Spark plug, alumina based insulator for spark plug and production process for same insulator

ABSTRACT

In insulator including alumina as a main component, a Na component in the insulator is set in the range of 0.07 to 0.5 wt % as Na 2 O. While the Na component content is as high as to have conventionally been regarded to be beyond the common sense, with this range of the Na component content, insulation resistance, mechanical strength and the like at high temperature are unexpectedly not reduced and an insulator with performances comparable to those of an insulator from conventional low soda alumina lower in Na component content than the above described range can be obtained. As a result, medium soda alumina and regular soda alumina that are much lower in cost than conventionally used low soda alumina can be used instead of the low soda alumina, so that dramatic reduction in production costs of insulator  2  for a spark plug  100  and in addition, of the spark plug  100  using the insulator  2  are realized.

FIELD OF THE INVENTION

[0001] The present invention relates to a spark plug, alumina basedinsulator used therein and a production process for the same insulator.

BACKGROUND ART

[0002] In a spark plug that is used for an internal combustion enginesuch as an automobile engine, alumina (Al₂O₃) based material excellentin heat resistance has been used as insulator therein from old times.Alumina as raw material for such insulator as described above hasgenerally been produced by Bayer process (hereinafter referred to asBayer alumina).

[0003] Bayer process is a process in which alumina is extracted frombauxite in a wet condition, which is naturally occurring aluminum ore,wherein an aqueous solution of caustic soda (NaOH) in a comparativelyhigh concentration is employed as an extracting reagent. Therefore,since thus obtained Bayer alumina includes a Na component (sodacomponent) to a considerable level, a soda removal treatment has oftenbeen applied before use on as-needed basis. Alumina is classifiedaccording to levels of soda removal into three classes which are shownas follows with respective common names: low alumina soda of a Nacomponent content less than 0.1 wt %, medium soda alumina of a Nacomponent content of the order of 0.1 to 0.2 wt % and regular sodaalumina of a Na component content more than the medium soda level, forexample of the order of 0.2 wt % or more.

[0004] Since a Na component included in alumina shows a nature of highionic conductance, there arise inconvenience when its content is inexcess: insulation resistance, especially insulation resistance at ahigh temperature of 500° C. or higher is reduced or a mechanicalstrength at a high temperature is deteriorated. Therefore, alumina basedinsulator used in a spark plug, which has a Na component at as small acontent as possible, has conventionally been considered to be the bestof its kind and it has been common sense that a content of the Nacomponent is set at a value as small as 0.05 wt % or less.

[0005] It is, as described above, indispensable to use alumina, as rawmaterial, of a low soda family in order to produce alumina basedinsulator low in Na component content. Alumina of a low soda family isexpensive because of a cost increase corresponding to a soda removalprocess step conducted out of necessity and has an aspect that thealumina is not necessarily desirable from the viewpoint of a rawmaterial cost. In recent years, however, in company with developmenttoward a high powered automobile engine, even insulator for a spark plugwith a higher withstand voltage and higher heat resistance has come tobe demanded. As a result, the common sense in regard to a low content ofa Na component in insulator has increasingly been firmer and the presentsituation is such that there is established a notion that a costincrease to some extent caused by use of low soda alumina has to beunavoidably accepted.

[0006] It is an object of the present invention to provide a spark plug,in which insulator is used that is available at as low a cost as beyonda so far established common sense associated with a Na component contentwhile no inferiority is found in its performance compared with existinginsulators, the insulator and a production process for the insulator.

DISCLOSURE OF INVENTION

[0007] A spark plug of the present invention comprises: a metal shellarranged outside a center electrode; a ground electrode arranged so asto be opposed to the center electrode, an end of the ground electrodebeing connected to the metal shell; and an insulator, which residesbetween the center electrode and the metal shell, and which insulatorsurrounds the outside of the center electrode, wherein

[0008] the insulator includes alumina as a main component and furtherincludes a Na component in the range of 0.07 to 0.5 wt % as a valueconverted into Na₂O, and

[0009] insulation resistance that is measured by passing a currentthrough the insulator between a terminal metal member and the metalshell while the entire spark plug is held at about 500° C. is 200 MΩ orhigher.

[0010] A production process for alumina based insulator for a spark plugof the present invention in order to produce an insulator used in theabove described spark plug is such that alumina powder in use includes aNa component in the range of 0.07 to 0.65 wt % as a value converted intoNa₂O and a Na component in the surface layers of particles of the powderis in the range of 0.01 to 0.2 wt % as a value converted into Na₂O; theraw material powder including such alumina powder as a main component isformed into a green (a powder compact) prescribed shape of an insulator;and then the green is sintered to obtain an insulator including aluminaas a main component and a Na component in the range of 0.07 to 0.5 wt %.It should be noted that the term a Na component content hereinaftermeans a content of Na₂O equivalent to the Na component content in thefollowing description unless otherwise specified.

[0011] The present inventors have conducted a serious study on an Nacomponent content in insulator and have discovered facts that, byadoption of a production process specific to the present, with aluminapowder, as raw material, in a high compositional range of a Na componentcontent that has been regarded to be of no common sense: in a concretemanner of description, with alumina powder containing a Na componentcontent in the range of 0.07 to 0.65 wt % as a value converted into Na₂Oand a Na component content in the range of 0.01 to 0.2 wt % as a valueconverted into Na₂O in the surface layers of particles of the powder,there can be obtained insulator whose performance is comparable to aconventional insulator containing a lower Na component content than theabove described range of the insulator, while insulation resistance andmechanical strength at high temperature is not reduced, unexpectedly. Ina detailed manner of description about the performance, a spark plugconstituted of the insulator can secure insulation resistance of 200 MΩor more which has conventionally been regarded to be impossible toacquire for the Na component content described above, wherein theinsulating resistance is measured in such a manner that the entire sparkplug is kept at about 500° C. and a current is made to pass through theinsulator between a terminal metal member and a metal shell.

[0012]FIG. 10 shows an example of a measuring system. Not only a DCconstant-voltage power supply (for example, a power supply voltage 100V)is connected to a terminal metal member 13 side of a spark plug 100, buta metal shell 1 side is grounded. The spark plug 100 thus outfitted isplaced into a heating furnace to heat the spark plug 100 at 500° C. anda current is made to pass through the spark plug 100. For example, acase is considered where a current measurement resistor (a resistancevalue is Rm) is used to measure a value of an applied current Im, thenan insulation resistance Rx to be measured can be obtained by a formula(VS/Im)−Rm, where VS is an applied voltage to pass the current (in thefigure, the applied current value Im is measured from an output of adifferential amplifier that amplifies a voltage difference between bothends of the current measurement resistor).

[0013] A first constitution of insulator for a spark plug of the presentinvention is such that the insulator includes alumina as a maincomponent, further includes a Na component in the range of 0.07 to 0.5wt % as a value converted into Na₂O and has an insulation withstandvoltage of 35 kV/mm or higher at 20° C. When alumina powder including aNa component in the range of 0.07 to 0.65 wt % as a value converted intoNa₂O and a Na component in the range of 0.01 to 0.2 wt % as a valueconverted into Na₂O in the surface layers of particles of the powder isused, there can be realized insulator having an insulation withstandvoltage comparable with that of a conventional insulator whose Nacomponent content is lower than the above described range in aluminapowder of the present invention.

[0014] An insulation withstand voltage of the insulator assembled in aspark plug can be measured in the following way: As shown in FIG. 9(a),a ground electrode is removed from a metal shell 1 of a spark plug 100and an opening side of the metal shell 1 in this state is immersed inliquid insulator medium such as silicon oil and thereby, the insulator 2and the metal shell 1 are insulated from each other by the liquidinsulator medium with which a space interposed between the outer sidesurface of the former and the inner side surface of the latter isfilled. In this state, not only is an AC high voltage or a pulse highvoltage applied between the metal shell 1 and a center electrode 3 by ahigh-tension power supply, but voltage wave forms of the applied voltage(which is dropped at a proper magnification by a voltage divider) arerecorded by an oscilloscope or the like.

[0015] Then, as shown in FIG. 9(b), not only is a voltage (bulkdielectric breakdown voltage) VD when bulk dielectric breakdown throughthe insulator 2 arises is read from a voltage wave form, but a value ofVD/LD as an insulation withstand voltage is obtained from a thickness LDof the insulator 2 at a penetration hole caused by bulk dielectricbreakdown and the bulk dielectric breakdown voltage VD, wherein aposition of the penetration hole is defined as a center of an openingthereof formed on an outer surface of the insulator 2. The thickness LDof the insulator 2 at a penetration hole is defined such that, as shownin FIG. 9(c), a section that is perpendicular to a center line O as anaxis of the insulator 2 including the center O_(G) of the opening istaken up for consideration, a straight line P is drawn through the twopoints O_(G) and O on the section to obtain an intersection K with aninner side surface of the insulator 2, and then a line segment KO_(G) isobtained and measured on its length to determine the thickness LD.

[0016] Below, further detailed description will be made of a spark plugand alumina based insulator used therein. First of all, by setting a Nacomponent content in the insulator in the above described range, aluminapowder as raw material of a Na component content as high as 0.07 to 0.65wt % as a value converted into Na₂O can be adopted (if the contentexceeds 0.65 wt %, a Na component content in the insulator obtainedtherefrom cannot be suppressed to be equal to or less than 0.5 wt %). Asa result, medium soda alumina, regular soda alumina and the like whichcan be acquired at a much lower cost can be adopted instead of low sodaalumina conventionally used, with the result that dramatic reduction inproduction costs of not only insulator for a spark plug but also a sparkplug using the insulator can be realized.

[0017] In order to confine a Na component content in the insulator toless than 0.07 wt %, alumina powder low in Na component content such aslow soda alumina has to be used, which makes it impossible to securesuperiority over conventional insulator from the viewpoint of rawmaterial cost. On the other hand, when a Na component content exceeds0.5 wt % in the insulator, insulation resistance of the insulator isinsufficient, which causes a withstand voltage performance required forthe insulator for a spark plug not to be satisfied. A Na componentcontent in the insulator is more desirably selected to be in the rangeof 0.07 to 0.25 wt %. Aluminum powder as raw material of a Na componentcontent in the range of 0.07 to 0.3 wt % as a value converted into Na₂Ois desirably used.

[0018] As alumina powder used in production of the above describedinsulator, alumina powder produced by Bayer process can be used. WhileBayer process has publicly been known, the process will be outlined forbetter understanding below: At first, bauxite which is a naturallyoccurring aluminum ore is pulverized into powder and then the powder issubjected to pressure extraction in an aqueous solution of caustic soda.An alumina component in the ore (for example, gibbsite or boehmite asalumina based ore) is dissolved as sodium aluminate in the solutionaccording to the following chemical reaction formula (1) on the basis ofan amphoteric character of Al element and separated from insolubleresidue (red sludge) such as Fe₂O₃, SiO₂ or TiO₂.

Al(OH)₃(sol)+NaOH(aq)

NaAl(OH)₄(aq)  (1)

[0019] While extraction conditions are different according to a kind ofalumina based ore, typically, for example, temperature is in the rangeof 120 to 300° C. and concentration of caustic soda is in the range 130to 380 g/l. The insoluble residue is filtered out from the solution toseparate an extract, the extract is added with seed alumina and aconcentration of caustic soda is reduced to a concentration in the rangeof 90 to 160 g/l. With such reduction in concentration, the extract issubjected to a reverse reaction of the formula 1 at temperature in therange of 35 to 80° C. for a period in the range of 30 to 60 hours toprecipitate and deposit aluminum hydroxide in a solid state. Thusobtained aluminum hydroxide is separated from the solution and isfurther washed, if necessary, and thereafter, the washed hydroxide iscalcined to produce Bayer alumina.

[0020] The present inventors have further conducted a serious study andfound that while alumina power produced by Bayer process contains a Nacomponent in the interior each of powder particles, a surface region ofeach powder particle has a Na component higher in concentration becauseof treatment by the caustic soda solution. Such a Na component in thesurface regions of the powder particles causes a problem: The Nacomponent in each of the surface regions of the powder particles forms aglassy phase by melting together with sintering aids (additive elementcomponent described later) in sintering. The glassy phase is associatedwith a problem since the phase decreases en electric resistivity due toformation of a solid solution of the Na component and such a solidsolution serves as a conductive channel, which causes decrease ininsulation resistance and reduction in insulation withstand voltage.

[0021] The present inventors have further conducted a serious study inlight of such a problem and as a result, have found that alumina powderto be preferably used is one whose particles have a Na component in therange of 0.01 to 0.2 wt % as a value converted into Na₂O in theirsurface regions. If alumina powder particles having the Na component inthe surface regions more than 0.2 wt % is used, there is a chance thatthe insulator obtained therefrom has the insulation resistance andinsulation withstand voltage both short of respective requirements. Inorder to confine the Na component content in the surface region of eachof powder particles (hereinafter referred to as surface Na amount) toless than 0.01 wt %, there are considered two ways, but any of both hasa fault: in one way, alumina low in Na component content, such as lowsoda aluminum, eventually has to be used, or in the other way, aluminapowder high in Na component content is used and therefore, a removalprocess for the Na component in the surface region of each of the powderparticles is complex or requires a long time period, which causessuperiority over conventional insulator with respect of raw materialcost to be lost. In this connection, a Na component content in theparticle surface regions are desirably in the range of 0.01 to 0.15 wt%, or more desirably in the range 0.01 to 0.1 wt%.

[0022] A Na component content in the surface region of each of powderparticles (surface Na amount) means a value measured in the followingway: At first, a total content in wt % of a Na component in aluminapowder which is an object to be measured is measured by ICP atomicemission spectrometry, a chemical analysis or the like and measurementsare converted to values in Na₂O, wherein the conversion value isindicated by W_(Na1). Then, 100 g of alumina powder is kept immersed in100 ml of a distilled water at a constant temperature of 90° C. for 1 hrwith no stirring. Thereafter, the alumina powder is separated forrecovery from the distilled water and the Na component content in wt %is again measured and the measurement is converted to a value in Na₂O,wherein the conversion value is indicated by W_(Na2). Then a value ofW_(Na1)-W_(Na2) in wt % is calculated and the value is regarded as acontent of the Na component existent in the surface region of each ofpowder particles.

[0023] While Bayer alumina powder is produced by calcining aluminumhydroxide obtained by extraction, a Na component content in the surfaceof each of powder particles is considerably different according to aproduction process. For example, when aluminum hydroxide is calcined, avariety of measures are taken so that a Na component liberated fromalumina particles is not diffused back to the alumina particle side. Asa typical measure, there has been known a process in which chlorine gasis made to flow in an atmosphere of calcination and liberated sodiumcomponent is fixed in the form of sodium chloride. However, since, inthis process, sodium chloride as a reaction product is remained on thesurfaces of alumina particles, therefore even after a water washingprocess of an ordinary level, the sodium chloride is still continued tobe remained at a considerable level. On the other hand, in anothermeasure, aluminum hydroxide particles are calcined in mixture withsilica particles coarser than the aluminum hydroxide particles are,thereby, a Na component liberated in the calcination is made to beabsorbed by the silica particles and the silica particles are lastlyseparated off by a sieve or another means. It has well known that aresidual amount of the Na component in the surfaces of alumina particlesthus treated is considerably decreased. Accordingly, in a case whereBayer alumina powder is used, the alumina powder has to be selected sothat not only a total Na component but also the surface Na amount arerespectively included in the ranges defined for a production process ofthe present invention. If the surface Na amount of power particlesunavoidably exceeds the defined range described above, it is importantthat the powder is washed by water (or acid cleaning) on an as-neededbasis so that the surface Na amount is adjusted to fall within the abovedescribed range and then thus washed powder is used.

[0024] An Al component content of the insulator converted to acorresponding equivalent value in Al₂O₃ (hereinafter referred to asW_(Al)) is preferably adjusted in the range of 85 to 95 wt %. If W_(Al)is less than 85 wt %, high temperature characteristics of a mechanicalstrength and a withstand voltage of the insulator are sometimesinsufficient. The W_(Al) is desirably set to be 90 wt % or more.However, if W_(Al) exceeds 98 wt %, a content of sintering aids isexcessively reduced on a relative basis and thereby the insulator aftersintering is difficult to be obtained in a highly densified state; forexample, if the insulator in a more densified state is tried to beobtained, then temperature increase in sintering cannot be avoided,which entails grain sizes of alumina particles constituting theinsulator to grow to be larger with the result of inconvenience such asdegradation of a mechanical strength, contrary to expectation. Hence, aW_(Al) is preferably adjusted in content equal to or less than 98 wt %.

[0025] Then, a second constitution of an insulator of a spark plug ofthe present invention is such that the insulator includes alumina as amain component, further includes a Na component in the range of 0.07 to0.5 wt % as a value converted into Na₂O, wherein an Al component isincluded in the range of 95 to 98 wt % as a value converted into Al₂O₃(W_(Al)). The insulator can be obtained with its leveled-up excellencyin insulation resistance or withstand voltage characteristics byadjusting W_(Al) in the high alumina compositional range of 95 to 98 wt%, though a Na component in the above described compositional range isincluded. Furthermore, when alumina powder as raw material with asurface Na amount in the above described range is used, a Na content ina glassy layer is reduced and the insulator with a higher withstandvoltage can be realized.

[0026] A structure of the insulator of the present invention isconstructed of an alumina based matrix phase particles of 99 wt % ormore in alumina content, as a main phase, and a glassy phase formed ingrain boundary regions of the alumina based matrix phase grains. In thiscase, when alumina powder with a low surface Na amount is used asdescribed above, a percentage of a Na content WG_(Na) (converted into avalue in Na₂O) present in the glassy phase of a total Na componentcontent in the insulator can be controlled in the range of 0.4 to 2 wt%, which is advantageous in order to secure an insulation resistance andan insulation withstand voltage and at the same time, is effective forreduction in raw material cost. A third constitution of an insulator ofa spark plug of the present invention is such that the insulatorincludes alumina as a main component, further includes a Na component inthe range of 0.07 to 0.5 wt % as Na₂O, wherein an Al component isincluded in the range of 85 to 98 wt % as a value converted into Al₂O₃(W_(Al)), wherein a structure of the insulator is constructed of analumina based matrix phase particles of 99 wt % or more in aluminacontent, as a main phase, and a glassy phase formed in grain boundaryregions of the alumina based matrix phase particles, and a Na componentcontent WG_(Na) present in a glassy phase is in the range of 0.4 to 2 wt%.

[0027] If WG_(Na) exceeds 2 wt %, an insulation resistance and aninsulation withstand voltage are sometimes both short of respectiverequirements. In order to reduce WG_(Na) to be less than 0.4 wt %,alumina powder low in a Na component content has to be used andtherefore, superiority in raw material cost over conventional insulatorcannot be held. It should be noted that the WG_(Na) that isapproximately calculated in the following way is adopted in the presentspecification: A surface of the insulator is polished, a structure ofthe insulator is observed on the polished surface with a scanningelectron microscope (SEM) and a picture of the structure is subjected toimage analysis to measure a area ratio of alumina based matrix(corresponding to a volume ratio), which is indicated by γA. Then, anaverage concentration by weight of a Na component in a glassy phase isidentified by publicly known micro-structure analytical methods such asan electron probe micro-analyzer (EPMA), energy dispersive spectrometer(EDS) or wavelength dispersive spectrometer (WDS) and obtained asNG_(Na) after conversion to a value in Na₂O. At this point, it isassumed that the insulator is material consisting of an alumina basedmatrix phase and a glassy phase only and further assumed that theinsulator is closely packed in an almost perfect manner by sintering.When an apparent density of the insulator measured a method based onArchimedes' principle or the like is indicated by ρ0 (in g/cm³ and adensity of alumina based crystal particle is indicated by ρ1 (=3.97g/cm³), a weight of a glassy phase MG per unit volume of the insulatoris given as follows:

MG=ρ0−ρ1·γA  (1)

[0028] Therefore, WG_(Na) can be calculated by the following formula:$\begin{matrix}\begin{matrix}{{WG}_{Na} = {{{MG} \cdot {NG}_{Na}} \times 100}} \\{= {{\left( {{\rho \quad 0} - {\rho \quad {1 \cdot \gamma}\quad A}} \right) \cdot N_{GNa}} \times 100\quad \left( {{wt}\quad \%} \right)}}\end{matrix} & (2)\end{matrix}$

[0029] It should be noted that an average of concentrations NG_(Na) of aNa component in the glassy phase is also preferably set in the range of0.4 to 2 wt % for the reason similar to the described above.

[0030] Then, in a case where insulator of the present invention isproduced using Bayer alumina powder, in common Bayer alumina powder,there is included almost none of alkali metal components other than a Nacomponent (hereinafter referred to as non-alkali metal component) exceptimpurities that are unavoidably included in the Bayer alumina powder.Accordingly, when such Bayer alumina powder is used, a total content ofnon-alkali metal components in the insulator obtained therefrom is equalto or less than 0.05 wt % as oxide as far as there is no intentionaladdition of such components. Since, of alkali metal components otherthan a Na component, Li or K has a chance to reduce a withstand voltageperformance, it is an advantageous approach to realize a high withstandvoltage insulator that Bayer alumina powder with no such componentsincluded is used.

[0031] A fourth constitution of the insulator of the present inventionis such that the insulator contains alumina as a main component, furtherincludes a Na component in the range of 0.07 to 0.5 wt % as a valueconverted into Na₂O and K and Li components equal to or less than 0.2 wt% in total content respectively as values converted into K₂O and Li₂O.When oxide components of Li, K and/or the like is inevitably added forthe purpose of adjustment of sintering temperature, it is desirable thata total content of the oxides combined respectively as K₂O and/or Li₂Ois confined to 0.2 wt % or less in order to secure a withstand voltageof the insulator.

[0032] The insulator of the present invention can contain one or moreadditive element components selected from the group consisting of Si,Ca, Mg, Ba, Zn, B and Na components at a total content in the range of0.1 to 15 wt % respectively as values converted into SiO₂, CaO, MgO,BaO, ZnO, B₂O₃ and Na₂O. Raw material powders to produce such aninsulator are each prepared by mixing a total of 0.1 to 15 parts byweight of additive element based raw materials including one or moreselected from the group consisting of Si, Ca, Mg, Ba, Zn and Brespectively as values converted into SiO₂, CaO, MgO, BaO, ZnO and B₂O₃into 85 to 98 parts by weight of alumina powder.

[0033] As additive element based raw material, for example, oxides(instead, complex oxides are allowed) of Si, Ca, Ba and Zn are used andbesides, there can be named a variety of kinds of powder of inorganicraw materials such as hydroxides, carbonates, chlorides, sulfates,nitrates and phosphates. The kinds of inorganic raw material powder arenecessary to be each used in a chemical form which can be transformedinto oxides by calcination. In a case of a B component, there can benamed: di-boron tri-oxide (B₂O₃), ortho-boric acid (H₃BO₃) and a variouskinds of other boric acids, and besides, borates of Al, which is a maincomponent element, and borates of Ca, Ba, Zn and the like.

[0034] The above described element components each are molten andproduces a liquid phase in the calcination process and function as asintering aid to accelerate formation of a densified state. If a totalcontent (hereinafter referred to as W1) as the above described oxides isless than 0.1 wt %, the sintered body in a densified state is difficultto be attained and a high temperature mechanical strength and a hightemperature withstand voltage performance are both short of respectiverequirements. On the other hand, if W1 exceeds 15.0 wt %, a hightemperature strength of the insulator is deteriorated. Therefore, atotal content W1 of additive element components is preferably set in therange of 1 to 15 wt %, or more desirably in the range of 3.0 to 10.0 wt%.

[0035] In the insulator of the present invention, there are also moredesirable embodiments associated with additive element components inorder to sufficiently secure a withstand voltage performance or amechanical strength at high temperature while maintaining a Na componentat a comparatively high concentration. A fourth constitution of theinsulator of the present invention in light of this point is such thatthe insulator includes one or more selected from the group consisting ofSi, Ca, Mg, Ba, Zn and B components at a total content of 60 wt % ormore respectively as values converted into SiO₂, CaO, MgO, BaO, ZnO andB₂O₃ of a remaining weight after excluding a weight as Al₂O₃ of the Alcomponent from a total weight. With the additive elements added, a moreadvantage can be enjoyed since a flowability of a glass component thatis formed in the sintering is increased, which enables the insulatorwith smaller defects such as pores to be realized.

[0036] The number of pores each having a size equal to or larger than 10μm that are observed in a sectional structure of an insulator isdesirably to be equal to or less than 100 as average counts per 1 mm² ofthe section. With such a number of the pores in the insulator, theinsulator can secure a better withstand voltage performance at hightemperature. At this point, the term a size of a pore is defined asshown in FIG. 8; When two parallel lines A and B are drawn so as to bein contact with an outer peripheral line of a section of a pore and notto cross the outer peripheral line, the maximum distance d betweenparallel lines A and B is determined, while positions of the parallellines A and B are geometrically changed relative to the outer peripheralline of the pore.

[0037] As one example, a composition can be shown in which the insulatorincludes one or more selected from the group consisting of Si, Ca and Mgcomponents at a total content of 60 wt % or more respectively as valuesconverted into SiO₂, CaO and MgO of a remaining weight after excluding aweight as Al₂O₃ of the Al component from a total weight.

[0038] On the other hand, a high temperature strength of an insulatorcan further be increased by mixing a Ba component and a B component intoa bulk. A BaO component may preferably be incorporated in the contentrange of 0.02 to 0.80 wt % as BaO (hereinafter referred to as W_(BaO)).If W_(BaO) is less than 0.02 wt %, an effect of BaO incorporation toimprove a high temperature strength is not conspicuous. On the otherhand, if W_(BaO) exceeds 0.80 wt %, a high temperature strength isdeteriorated. W_(BaO) is desirably adjusted to be in the range 0.15 to0.50 wt %. On the other hand, a B component is preferably included inthe range of 0.01 to 0.75 wt % as B₂O₃ (hereinafter referred to as W_(B)₂ _(O) ₃ ). If W_(B) ₂ _(O) ₃ is less than 0.01 wt %, an effect of a Bcomponent incorporation to improve a high temperature strength is notconspicuous. On the other hand, if W_(B) ₂ _(O) ₃ exceeds 0.75 wt %, ahigh temperature strength is deteriorated. W_(B) ₂ _(O) ₃ is desirablyadjusted to be in the range 0.15 to 0.50 wt %.

[0039] The above described Ba component and B component may be usedsingly or in combination and when both are simultaneously used, a totalcontent is preferably in the range of 0.2 to 1.2 wt % as oxidesdescribed above.

[0040] A bending strength of insulator of the present invention ispreferably secured to be 350 MPa or more. If a bending strength is lessthan 350 MPa, a breakdown or the like due to shortage of strength is aptto take place when a spark plug using such an insulator is mounted to acylinder head at a holding section. The bending strength is desirably tobe 400 MPa or more. Incidentally, the term a bending strength means athree point bending strength (a span length 20 mm) measured at roomtemperature in conformance with a method described in JIS R1601 (1981)(Testing method for flexural strength (modulus of rapture) of highperformance ceramics).

[0041] In order to make an additive element component functioneffectively as a sintering aid, it is important to produce a glassyphase good in flowability neither too much not too less at a prescribedsintering temperature that is set to lower than that of Al₂O₃. It ismore often effective when several kinds of additive components are mixedin use than when the additive components is singly used. For example,when first additive element components of the above five kinds are allin the respective forms of oxides, the additive element components inalumina powder before sintering desirably include a Si component in therange of 1.50 to 5.00 wt % as a value converted into SiO₂, a Cacomponent in the range of 1.20 to 4.00 wt % as a value converted intoCaO, a Mg component in the range of 0.05 to 0.17 wt % as a valueconverted into MgO, a Ba component in the range of 0.15 to 0.50 wt % asa value converted into BaO and a B component in the range of 0.15 to0.50 wt % as a value converted into B₂O₃.

[0042] Together with the additive element components, one or moreelement components selected from the group consisting of Sc, V, Mn, Fe,Co, and Zn can be included in the insulator as auxiliary additiveelement components at a total content in the range of 0.1 to 2.5 wt %(desirably in the range of 0.2 to 0.5 wt %) as respective oxides. Withsuch auxiliary additive element components in use, an effect to improvewithstand voltage performance characteristics at high temperature isespecially exerted. Of the auxiliary additive element components, a Mncomponent exerts an effect to improve withstand voltage performancecharacteristics especially in a conspicuous manner and preferred to beused in working of the present invention.

[0043] While an effect to improve withstand voltage characteristics canalso be expected when a Mn component (or MnO) is singly used, the effectcan further be conspicuous by adding a Cr component (or Cr₂O₃). In thiscase, when a Mn component content as a value converted into MnO isindicated by W_(Mn) in wt % and a Cr component content as a valueconverted into Cr₂O₃ is indicated by W_(Cr) in wt %, it is preferredthat Mn and Cr components are mixed so that W_(Mn)/W_(Cr) is in therange 0.1 to 10.0. If a value is outside the range of W_(Mn)/W_(Cr), theabove described double addition effect is not necessarily conspicuous.Further, when only Mn and Cr components are used as auxiliarycomponents, it is preferred to adjust W_(Mn)+W_(Cr) is in the range of1.2 to 2.5 wt %, or desirably in the range of 0.2 to 0.5 wt %.

[0044] According to a study of the present inventors, it is found thatwhen Mn and Cr components are simultaneously added, a Mn—Al basedcomplex oxide phase (for example Mn—Al based spinel phase) of a highmelting point is formed in the insulator. While a glassy phase caused bya sintering aid is formed in a surrounding manner around an aluminabased matrix phase in the insulator, it is said that the glassy phase isgenerally higher in conductivity than the matrix phase and serves as aconductive route for current with ease when breakdown in insulatoroccurs. However, it is considered that an insulator of the presentinvention in which the Mn and Cr components are simultaneously added hasa state in which the complex oxide phase of high melting point isdispersed in the glassy phase and the conductive route is disconnectedor made to go round by the complex oxide phase and thereby, a withstandvoltage in dielectric break-down is increased.

[0045] While the additive element components and the auxiliary additiveelement components are considered to be mainly contained in the chemicalforms of oxides, in many cases existing forms of oxides cannot directlybe identified for reasons such as formation of an amorphous glassyphase. In this case, if a total content of the additive elementcomponents is in the ranges expressed in oxides, the insulator isregarded to fall in the scope of the present invention. It can beconfirmed by the following three methods, singly or in combination,whether or not an Al component and an additive element component areincluded in a insulator:

[0046] (1) By X-ray diffraction, it is confirmed whether or not adiffraction pattern that reflects a crystal structure of a particularoxide can be attained.

[0047] (2) When a compositional analysis in a section of a materialspecimen is conducted by publicly known micro-structure analyticalmethods such as EPMA, EDS or WDS, it is confirmed whether or not an Alcomponent or an additive element component and an oxygen component aresimultaneously detected. If simultaneously detected, it is regarded thatthe Al component or the additive element component is present in theform of an oxide.

[0048] (3) The number of valencies of an atom or an ion of an Alcomponent or an additive element component is analyzed by publicly knownmethods such as X-ray photoelectron spectroscopy (XPS), Auger electronspectroscopy (AES). If the components are present in the forms ofoxides, the valency number is measured as a positive number.

[0049] An insulator of a spark plug of the present invention is producedin such a manner that, as described above, raw material powder is formedinto a green with a prescribed shape of an insulator and the green issintered. In this case, particles of alumina powder serving as a maincomponent of raw material powder are preferably in the range of 1 to 5μm as average particle diameter. If the average size exceeds 5 μm, aconsiderably high temperature is required in order to sufficientlyadvance a densification of a sintered body. However, in some instances,even if a sintering temperature is considerably set high, thedensification of the sintered body is not advanced up to a sufficientlyhigh level and thereby, a high temperature strength of the insulator isshort of a required level and an insulation withstand voltage isinsufficient. Alumina powder with particles in the range of 1 to 3 μm ismore desirably used.

[0050] In this case, it is preferred that an average size of crystalgrains in an alumina based matrix phase in the green is preferably inthe range of 2 to 20 μm (or desirably in the range of 5 to 10 μm). Theterm a crystal grain's diameter here is defined such that, inconformation with the way to determine a pore size shown in FIG. 8 andwith reference to FIG. 8, when two parallel lines A and B are drawn soas to be in contact with an outer peripheral line of a crystal grainthat is observed on a polished surface of the insulator and not to crossthe outer peripheral line, the maximum distance d between parallel linesA and B is determined, while positions of the parallel lines A and B aregeometrically changed relative to the outer peripheral line of thecrystal grain and the term an average grain diameter means an average ofgrain diameters of many crystal grains measured in this way.

[0051] Now, alumina powder of a Na content as high as of 0.07 to 0.65 wt% is used in raw material for production of an alumina based insulatorof the present invention. The present inventors have been a seriousstudy and have found facts that the raw material using alumina powder ofsuch a high Na content as this is resulted in a green more fragile thanthat in a case of alumina powder of a low Na content and for example, inpress forming, defects such as cracks and edge collapses occur in agreen and thereby a forming performance is not necessarily good.

[0052] Why alumina powder of a high Na content such as medium sodaalumina and regular alumina have not been used in an insulator for aspark plug is naturally considered to be, as a main reason, thatreduction in insulation resistance and mechanical strength of aninsulator obtained, as described above, is too much worried about, whichforces it be of common sense to use low soda alumina. However, a problemof reduction in production yield due to poor forming performance on theraw material powder is imagined to be paralleled to the problem causedby raw material powder with a high Na component content, as a greatobstacle for industrialization.

[0053] The present inventors have further conducted a serious study inlight of such a problem, completed a series of production processes thatwill be described below in order to improve forming performance of rawmaterial powder and thereby, established processes for industriallyproviding alumina based insulator for a spark plug of the presentinvention. Below, detailed description will be made of the processes.

[0054] Main features of a process described above are that not only is abinder in a prescribed amount mixed in raw material powder to preparepreform-use powder but a proper acidic component is added to thepreform-use powder to adjust a pH value of the preform-use powder so asto be lowered, thereafter, the preform-use powder after the pHadjustment is subjected to forming to produce a green and then the greenis sintered to obtain an insulator.

[0055] It is found by the present inventors that much of a Na componentoriginated from alumina powder in the preform-use powder is existent inthe forms of a strongly basic Na compound such as Na₂O₃ or NaOH andthereby, when a pH value of the preform-use powder is shifted toward abasic region side, a forming performance of the powder is deteriorated.According to the above described process, the pH thus shifted to thebasic side is lowered to a value in the proper range by adding theacidic component, which greatly improves the forming performance of thepreform-use powder and thereby, defects such as cracks and edgecollapses are conspicuously suppressed from occurring, so that a yieldof production can be improved by a great margin.

[0056] In this case, the pH of the preform-use powder is preferablyadjusted in the range of 6 to 10. If a value of the pH exceeds 10 or islowered to less than 6, in any of the cases, the forming performance ofthe preform-use powder is deteriorated, which leads to reduction of aproduction yield. The pH is more desirably adjusted in the range of 7 to9.

[0057] As acidic components, there can be used one or more selected fromthe group consisting of inorganic acids such as boric acid, colloidalsilica, carbonic acid and phosphoric acid; organic acids such as citricacid, oxalic acid, tartaric acid and acetic acid; and a salt of a weakbase and a strong acid such as ammonium sulfate or ammonium nitrate. Forexample, when a boric acid (for example ortho-boric acid) is used, a Bcomponent in the acid is incorporated into the insulator and an effectto improve a high temperature strength of the insulator can also beexpected as described.

[0058] Preform-use powder for alumina based insulator for a spark plugis generally prepared by mixing a flux mainly composed of water into rawmaterial powder to form slurry, in which case, a hydrophilic binder isused. However, a problem arises since a forming performance of apreform-use powder mixed with a hydrophilic binder is especiallysensitive to a change in pH and as the pH is increased to the basic sideregion, a forming performance is rapidly deteriorated. The reason why isestimated that many of hydrophilic binders produces a caking propertymainly due to a hydrogen bond between polar molecules and as the pH ishigher, the hydrogen bond is hindered by actions of basic ions andthereby a forming performance is deteriorated. Any way, by adding anacidic component, a value of the pH is corrected downward and a formingperformance of preform-use powder in which a hydrophilic binder isincluded is improved in an extremely effective manner and as a result, aproduction yield can be increased.

[0059] In the mean time, as hydrophilic binders in use, there can benamed: polyvinyl alcohol (PVA), dextrin, polyvinyl pyloridone, starch,carboxymethyl-cellulose alkaline salt (for example,carboxylmethylcellulose sodium (CMC)) and water soluble acrylic resin(for example, polyacrylic acid salt based resin).

[0060] The above described process can be performed through thefollowing steps in a concrete manner of description: That is, a waterbased solvent and a hydrophilic binder in a prescribed amount areblended to raw material powder (including alumina powder), and the fluxand the powder are mixed with each other to form slurry and on the otherhand, an acidic component is added to the slurry to adjust a pH value ofthe slurry so as to be in the range of 6 to 10 (desirably in the rangeof 7 to 9). The slurry is jet-atomized and dried to produce granules ofthe preform-use powder and the granules are subjected to press formingto obtain a green. A preferred press forming method is a cold isostaticpress (CIP) and especially a more preferred press forming method is arubber press forming method in which a rubber mold is used, from theview point of production with high efficiency and high yield of a long,narrow insulator for a spark plug.

[0061] In this case, it is desirable that 100 parts by wt of granules isadded with 0.5 to 2.0 parts by wt (desirably 0.7 to 1.3 parts by wt) ofwater so that breaking of the granules into powder is accelerated duringthe press forming, and after the addition of water, the granules aresubjected to press forming. At this point, when the pH adjustmentthrough the addition of an acidic component to the granules is notconducted, by the addition of water a strongly basic Na compound such asNa₂O₃ and NaOH included in alumina powder is ionized to produce a basicion, which adversely works so as to suppress a function of a hydrophilicbinder with the result that forming performance is deteriorated withease. However, by pH adjustment through the addition of an acidiccomponent, the forming can be performed with no problem even if water isadded to the granules.

[0062] In the mean time, a spark plug of the present invention can be ofa structure having an ignition portion, which provides a spark dischargegap, and which is fast held by one of a center electrode and a groundelectrode. In this case, the ignition portion can be made mainly of anoble metal alloy composed of one or more selected from the groupconsisting of Ir, Pt and Rh as a main component or components. With suchalloy, even when the spark plug is applied in a high powered internalcombustion engine, durability of the ignition portion can be improved toa great extent. For example, in a case where a Pt based alloy is used,Pt—Ni alloy (for example, Pt-1 to 30 wt % Ni alloy) can preferably beused. As alloy of Ir as a main component, the following alloy can beadopted, for example.

[0063] (1) An alloy composed mainly of Ir that contains 3 to 50 wt % Rh(50 wt % is not included) is used. With the alloy in use, consumption ofthe ignition portion caused by oxidation/volatilization of an Ircomponent at high temperature is effectively suppressed, which isresulted in realization of a spark plug excellent in durability.

[0064] If a Rh content in the alloy is less than 3 wt %, a suppressioneffect of oxidation/volatilization of Ir is insufficient and therebyconsumption of the ignition portion becomes easy, so that the durabilityof a spark plug is reduced. On the other hand, if a Rh content is equalto or more than 50 wt %, a melting point of the alloy is lowered and thedurability of the plug is reduced as well. As can be clear from theabove description, a content of Rh is preferably adjusted in the abovedescribed range and the content is desirably in the range of 7 to 30 wt%, more desirably in the range of 15 to 25%, and most desirably in therange of 18 to 22 wt %.

[0065] (2) An alloy composed mainly of Ir that contains 1 to 20 wt % ofPt is used. With the alloy in use, consumption of the ignition portioncaused by oxidation/volatilization of an Ir component at hightemperature is effectively suppressed, which is resulted in realizationof a spark plug excellent in durability. If a Pt content in the alloy isless than 1 wt %, a suppression effect of oxidation/volatilization of Iris insufficient and thereby consumption of the ignition portion becomeseasy, so that the durability of a spark plug is reduced. On the otherhand, if a Pt content is equal to or more than 20 wt %, a melting pointof the alloy is lowered and the durability of the plug is reduced aswell.

[0066] (3) An alloy composed mainly of Ir that contains 0.1 to 35 wt %Rh and further 0.1 to 17 wt % Ru is used. With the alloy in use,consumption of the ignition portion caused by oxidation/volatilizationof an Ir component at high temperature is effectively suppressed, whichis resulted in realization of a spark plug excellent in durability. If aRh content in the alloy is less than 0.1 wt %, a suppression effect ofoxidation/volatilization of Ir is insufficient and thereby consumptionof the ignition portion becomes easy, so that the durability of a sparkplug is reduced. On the other hand, if a Rh content is more than 35 wt%, a melting point of the alloy is lowered and resistance to sparkconsumption of the plug is reduced, which makes it impossible to securethe durability of a spark plug. Hence, a content of Rh is adjusted inthe above described range.

[0067] On the other hand, if a Ru content is less than 0.1 wt %, aneffect of addition of elements to suppress consumption ofoxidation/volatilization of Ir is insufficient. If a Ru content exceeds17 wt %, the ignition section is, contrary to expectation, apt to beaffected by more of spark consumption and as a result, the durability ofthe plug cannot be secured. Hence, a Ru content is adjusted in the abovedescribed range, desirably in the range of 0.1 to 13 wt % and moredesirably in the range of 0.5 to 10 wt %.

[0068] One of reasons by which resistance to consumption of the ignitionportion is improved by addition of Ru in the alloy is guessed that forexample, by addition of the component, an oxide layer stable and denseat high temperature is formed on alloy surface and thereby Ir oxide as asingle material which is very easy to be vaporized is fixed in the oxidelayer. It is further conjectured that the oxide film works as a kind ofpassive film and suppresses progress in oxidation of an Ir component.Since, in a condition of no addition of Rh, resistance tooxidation/volatilization at high temperature of the alloy is not muchimproved even with the addition of Ru, it is considered that the oxidefilm is an Ir—Ru—Rh based complex oxide or the like oxide and such anoxide is higher in density or adherence to an alloy surface than anIr—Ru based oxide.

[0069] If a total content of Ru is excessively increased, it isconjectured that spark consumption of Ir oxide progresses in thefollowing mechanism rather than by vaporization: A density of the oxidelayer formed and adherence thereof to an alloy surface is reduced andsuch a phenomenon is conspicuous when the total content exceeds 17 wt %.When a shock of spark discharge is repeated in a spark plug, it isimagined that the oxide film formed is easy to be separated from thealloy surface and thereby, a new metal surface is exposed to facilitatespark consumption.

[0070] By the addition of Ru, the following important effect canadditionally be achieved: That is, by the addition of Ru, even if a Rhcontent is greatly decreased, consumption resistance can sufficiently besecured and a high performance spark plug can be constructed at a lowcost compared with a case where an Ir—Rh binary alloy is adopted. Inthis case, a Rh content is preferably in the range of 0.1 to 3 wt %.

[0071] (4) An alloy composed mainly of Ir that contains at least one ofPt, Re and Pd in the range of 1 to 30 wt % and besides, Ru in the rangeof 1 to 30 wt % is used. When the ignition portion is constituted of analloy including Ir as a main component and further Pt, Re or Pd in theabove described range, not only is consumption due tooxidation/volatilization of an Ir component at high temperatureeffectively suppressed but also its workability in mechanical processingis dramatically improved by additional inclusion of Rh in the abovedescribed range. A chip can be a metal piece formed by applyingprescribed mechanical processing to an alloy mass of a prescribedcomposition obtained by mixing/melting of raw material, wherein the termmechanical processing means one or more selected from the groupconsisting of rolling, forging, shaping by cutting, dividing by cuttingand blanking.

[0072] If a Rh content is less than 1 wt %, an effect to improveworkability of the alloy in the mechanical processing cannotsufficiently be achieved and for example, breaking, cracking and thelike are apt to arise during the mechanical processing, leading toreduction of a yield rate of the effectively used in a product tostarting material. When a chip is produced by hot blanking or the like,consumption or damage of a tool such as blanking edge is easy to occur,thereby resulting in reduction in production efficiency. On the otherhand, if a Rh content exceeds 49 wt %, a melting point of an alloy islowered, which entails decrease in durability of a plug. Therefore, a Rhcontent is preferably adjusted in the range of 2 to 20 wt % andespecially, in a case where a total content of Pd and/or Pt is equal toor more than 5 wt %, an alloy becomes further fragile and therefore inthis case, unless Rh is added in a content equal to or more than avalue, a chip production by mechanical processing is extremelydifficult. In this case, additive Rh is required to be 2 wt % or more inthe broadest sense, but desirably 5 wt % or more, and more desirably 10wt % or more. It should be noted that in a case of a Rh content equal toor more than 3 wt %, Rh exerts not only an effect to improve theworkability in the mechanical processing, but, though sometimes, aneffect to suppress oxidation/ volatilization of an Ir component at hightemperature.

[0073] If a total content of Pt and/or Pd is less than 1 wt %, an effectto suppress oxidation/volatilization of Ir becomes insufficient and achip is easy to be consumed, which entails reduction in the durability.On the other hand, if the content exceeds 30 wt %, a problem arises,since a melting point of the alloy is lowered and the durability of theplug is likewise decreased (for example, in a case where Pd is singlyused) or an effect to suppress consumption of a chip cannot be as highas expected in consideration of increase in material cost due toincrease in content of expensive Pt and/or Pd. As can be seen from theabove description, a total content of Pt and/or Pd is preferablyadjusted in the above described range and desirably in the range of 3 to20 wt %.

[0074] Besides, in any of materials described in (1) to (3), a materialconstituting a chip can contain oxides (including double or multipleoxides) of a metal or metals belonging to the group 3A (so-called rareearth elements) and the group 4A (Ti, Zr and Hf) of the periodic tableof the elements in the range of 0.1 to 15 wt %. With the additionalinclusion, consumption of an Ir oxide through oxidation/volatilizationthereof is further effectively suppressed. If a total content of theoxides is less than 0.1 wt %, an effect to preventoxidation/volatilization of Ir cannot sufficiently be attained. On theother hand, if a total content of the oxides exceeds 15 wt %, thermalshock resistance of a chip is reduced and, for example, when the chip isfixed to an electrode by welding or the like, inconveniences such ascracking sometimes occurs. While as the above described oxides, Y₂O₃ ispreferably used, it is noteworthy in addition that La₂O₃, ThO₂, ZrO₂ andthe like can preferably be used.

BRIEF DESCRIPTION OF THE DRAWINGS

[0075]FIG. 1 is a sectional front view showing the whole of an exampleof a spark plug of the present invention;

[0076]FIG. 2 is a partly sectional front view of a main part of FIG. 1;

[0077]FIG. 3 is a further enlarged sectional view showing theneighborhood of an ignition portion of FIG. 1;

[0078]FIG. 4 is a vertical sectional view showing examples ofinsulators;

[0079]FIG. 5 is a front view showing the whole of another example of aspark plug of the present invention;

[0080] FIGS. 6 is a plan view of FIG. 5 and its alternate example;

[0081]FIG. 7 is a front view showing the whole of still another exampleof a spark plug of the present invention;

[0082]FIG. 8 is an illustration for a definition of a size of a pore ora crystal grain present in an insulator;

[0083] FIGS. 9 is a diagram and enlarged sectional view illustrating ameasuring method for an insulation withstand voltage;

[0084]FIG. 10 is an illustration for a measuring method for insulationresistance of a spark plug;

[0085]FIG. 11 is a sectional view illustrating a rubber press method;and

[0086]FIG. 12 is a perspective view showing an example of a greenobtained by a rubber press method and ways of generating defects in thegreen.

BEST MODE TO CARRY OUT THE INVENTION

[0087] Below, description will be made of embodiments of the presentinvention with reference to the accompanying drawings.

[0088] A spark plug 100 as an example of the present invention shown inFIGS. 1 and 2 comprises: a metal shell 1 in a cylindrical shape; aninsulator 2 that is fittingly inserted within the metal shell 1 so thata fore-end 21 thereof projects; a center electrode 3 that is provided inthe interior of the insulator 2 in a state in which a first ignitionportion 31 formed at a top of the fore-end of the center electrodeprojects; and a ground electrode 4, one end of which is welded to themetal shell 1, and which is arranged so as to be opposed to the fore-endof the center electrode. A second ignition portion 32 is formed on theground electrode 4 so as to be opposed to the first ignition portion 31and a spark discharge gap g is formed between the opposed ignitionsections 31 and 32.

[0089] A through hole is formed along an axial direction of theinsulator 2 and a metal terminal 13 is inserted and fixed from one endside thereof while the center electrode 3 is inserted and fixed from theother end side likewise. A resistor 15 is disposed between the metalterminal 13 and the center electrode 3 in the through hole 6. Both endsof the resistor 15 are electrically connected with the center electrode3 and the metal terminal 13 respectively by way of conductive glass seallayers 16 and 17 interposed therebetween. The resistor 15 is formed insuch a manner that glass powder and conductive material powder (and, asneeds come up, ceramic powder other than glass powder) are mixed witheach other and the mixed powder is sintered by a hot press or the liketo form a resistor composition mass. The metal terminal 13 and thecenter electrode 3 may be integrated as one body with one layerconductive glass seal layer while the resistor 15 is omitted.

[0090] The insulator 2 has the through hole 6 in the interior thereof tofittingly insert the center electrode 3 therethrough along an axial linethereof and the whole structure is constituted as an insulator of thepresent invention. That is, the insulator 2 is constituted as an aluminabased ceramic sintered body that, as a main component, comprisesalumina, wherein an Al component amounts to a content in the range of 85to 89 wt % (desirably in the range of 90 to 98 wt %) as a valueconverted into Al₂O₃, and further a Na component amounting to a contentin the range of 0.07 to 0.5 wt % (desirably in the range of 0.07 to 0.25w %).

[0091] As a more concrete composition, the following is exemplified:

[0092] a Si component in the range of 1.50 to 5.00 wt % as a valueconverted into SiO₂;

[0093] a Ca component in the range of 1.20 to 4.00 wt % as a valueconverted into CaO;

[0094] a Mg component in the range of 0.05 to 0.17 wt % as a valueconverted into MgO;

[0095] a Ba component in the range of 0.15 to 0.50 wt % as a valueconverted into BaO; and

[0096] a B component in the range of 0.15 to 0.50 wt % as a valueconverted into B₂O₃.

[0097] As shown in FIG. 1, the protrusion 2 e is formed in the shape ofa flange extending outwardly around the periphery of the insulator 2 atabout the middle point of its length. The insulator 2 is constructed ofthe forward side that is constituted of the side closer to the fore-endof the center electrode 3 (FIG. 1) from the protrusion 2 e with theprotrusion 2 e as a boundary and the backward side that is constitutedof the part contrary to the forward side. The backward side formed witha diameter smaller than that of the protrusion 2 e constitutes a bodysection 2 b, while the forward side is constructed of a first stemsection 2 g smaller in diameter than that of the protrusion 2 e and asecond stem section 2 i smaller in diameter than that of the first stemsection 2 g in the written order toward the fore-end. A glaze 2 d isapplied on the outside surface of the main body section 2 b and theouter surface in the back end portion next to the glazed surface isformed as a corrugation 2 c. An outer side surface of the first stemsection 2 g is formed in an almost cylindrical shape and an outer sidesurface of the second stem section 2 i is formed in a conical shapetapering toward the fore-end.

[0098] A diameter of a section of the center electrode 3 is smaller thanthat of the resistor 15. The through hole 6 of the insulator 2 isconstructed of a first part 6 a in an almost cylindrical shape throughwhich the center electrode 3 is inserted and a second part 6 b in analmost cylindrical shape of a diameter larger than that of the firstpart, which is disposed in the back side (the upper side in the figure)of the first part 6 a. As shown in FIG. 1, the metal terminal 13 and theresistor 15 are accommodated in the second part 6 b and the centerelectrode 3 is inserted through the interior of the first part 6 a. Anelectrode fixing protrusion 3 a is formed at the rear-end portion of thecenter electrode 3 while protruding outwardly from the outer sidesurface of the rear-end portion. The first part 6 a and second part 6 bof the through hole 6 is connected to each other in the interior of thefirst stem section 2 g and the protrusion resting surface 6 c is formedin a tapered surface or a concave surface at the connecting position ofboth parts 6 a and 6 b to receive the electrode fixing protrusion 3 a ofthe center electrode 3.

[0099] An outer side surface of the connecting section 2 h between thefirst stem section 2 g and the second stem section 2 i are formed in theshape of a step surface and the step surface works as a stopper forbeing extracted in the axial direction by engaging with a annular stepsection 1 c that is formed on an inner side surface of the metal shell 1as a metal shell side engaging section with a ring-like plate packing 63interposed therebetween. On the other hand, in a space between the innerside surface of the backward side opening section of the metal shell 1and the outer side surface of the insulator 2, there are provided: awire packing ring 62 which engages with a rear side peripheral surfaceof the protrusion 2 e; a filler layer 61 of talc or the like followingthe wire packing ring 62 in the backward direction; and a wire packingring 60 further following the filler layer 62 in the backward direction.The insulator 2 is pushed to the forward side and, in the state, theopening brim of the metal shell 1 of the metal shell 1 is caulkedinwardly while the opening brim of the metal shell 1 is opposed to thepacking ring 60 to form a caulking section 1 d, so that the metal shell1 is fixed to the insulator 2.

[0100] FIGS. 4(a) and 4(b) shows examples of the insulator 2. Dimensionsof parts of insulators are exemplified below:

[0101] a total length LI in the range of 30 to 75 mm;

[0102] a length L2 of the first stem section 2 g in the range of 0 to 30mm (wherein the connection section 2 f with the engaging protrusion 2 eis not included but the connection section with the second stem section2 i is included);

[0103] a length L3 of the second stem section 2 i in the range of 2 to27 mm;

[0104] an outer diameter D1 of the main body section 2 b in the range of9 to 13 mm;

[0105] an outer diameter D2 of the engaging protrusion 2 e in the rangeof 11 to 16 mm;

[0106] an outer diameter D3 of the first stem section 2 g in the rangeof 5 to 11 mm;

[0107] an outer diameter D4 of the base end portion of the second stemsection 2 i in the range of 3 to 8 mm;

[0108] an outer diameter D5 of the fore-end portion of the second stemsection 2 i in the range of 2.5 to 7 mm (wherein when an outer peripheryof the fore-end surface is rounded or chamfered, an outer diameter at abase end position of the rounded part or chamfered part in a sectionincluding the center line O as an axis is measured);

[0109] an inner diameter D6 of the second part 6 b of the through hole 6in the range of 2 to 5 mm;

[0110] an inner diameter D7 of the first part 6 a of the through hole 6in the range of 1 to 3.5 mm;

[0111] a thickness t1 of the first stem section 2 g in the range of 0.5mm to 4.5 mm;

[0112] a thickness t2 of the base end portion of the second stem section2 i in the range of 0.3 to 3.5 mm(a value in a direction perpendicularto the center line O as an axis); and

[0113] a thickness t3 of the fore-end portion of the second stem section2 i in the range of 0.2 to 3 mm (a value in a direction perpendicular tothe central line O as an axis, however, when an outer periphery of thefore-end surface is rounded or chamfered, an outer diameter at a baseend position of the rounded part or chamfered part in a sectionincluding the center line O as an axis is measured); and

[0114] an average thickness tA=t1+t2/2 of the second stem section 2 i inthe range of 0.25 to 3.25 mm.

[0115] In FIG. 1, a length L_(Q) of a portion 2 k projecting toward thebackward side of the metal shell 1 of the insulator 2 is in the range of23 to 27 mm (for example, of the order of 25 mm). A length LP measuredalong a sectional outline from a position corresponding to the rear endbrim of the metal shell 1 through the corrugation 2 c to the rear endbrim of the insulator 2 is in the range of 26 to 32 mm (for example, ofthe order of 29 mm).

[0116] The dimensions of the respective parts of an insulator 2 shown inFIG. 4(a) are, for example, such that L1=about 60 mm, L2=about 10 mm,L3=about 14 mm, D1=about 11 mm, D2=about 13 mm, D3=about 7.3 mm, D4=5.3mm, D5=4.3 mm, D6=3.9 mm, D7=2.6 mm, t1=3.3 mm, t2=1.4 mm, t3=0.9 mm andtA=1.2 mm.

[0117] The insulator 2 shown in FIG. 4(b) has slightly larger outerdiameters of the first shaft section 2 g and the second shaft section 2i than those in the case shown in FIG. 4(a). The dimensions of therespective parts are, for example, such that L1=about 60 mm, L2=about 10mm, L3=about 14 mm, D1=about 11 mm, D2=about 13 mm, D3=about 9.2 mm,D4=6.9 mm, D5=5.1 mm, D6=3.9 mm, D7=2.7 mm, t1=3.3 mm, t2=2.1 mm, t3=1.2mm and tA=1.7 mm.

[0118] In FIG. 1, the metal shell 1 is made with a metal such as lowcarbon steel or the like in the form of a cylinder and not onlyconstitutes housing of a spark plug 100 but is provided with a threadedportion 7 for mounting the plug 100 to an engine block not shown. Areference numeral 1 e indicates a tool engaging portion with which atool such as a spanner or a wrench engages and the portion has ahexagonal section in a plane perpendicular to the axis.

[0119] The main body sections 3 a and 4 a of the center electrode 6 andthe ground electrode 4 (FIG. 3) are made of Ni alloy or the like. A coreportion 3 b made of Cu, Cu alloy or the like in order to accelerate heatdissipation is embedded in the interior of the center electrode 3. Onthe other hand, the first ignition portion 31 and the second ignitionportion 32 opposed to the first one are made mainly of a noble metalalloy which is composed of one or more selected from the groupconsisting of Ir, Pt and Rh. As shown in FIG. 3, not only is the mainbody section 3 a of the center electrode 3 tapered toward the fore-endside, but also the fore-end surface is formed flat. Disk-like chips madeof a alloy composition constituting the ignition portions are placed ina superposing manner on the flat fore-end surface and a welded part W isformed by laser welding, electron beam welding, resistance welding orthe like to fix and form the first ignition portion 31. The secondignition portion 32 is formed likewise in such a manner that a chip isplaced on the ground electrode 4 at a corresponding position of thefirst ignition portion 31 and then a welded part is formed along a outerperiphery of a bonding surface to fix the chip. The chips can be madefrom an alloy obtained by mixing alloy components in a composition inthe above described ranges and thereafter melting the mixture, or from asintered body obtained by press forming and sintering powder of an alloyobtained by the mixing and melting or a powder mixture composed ofvarious kinds of single metal component powder in a prescribedcomposition. It should further be noted that at least one of the firstand second ignition portions may be omitted.

[0120] The insulator 2 is produced, for example, in a process as shownbelow: First of all, as raw material powder, Bayer alumina powder havinga Na component content in the range of 0.07 to 0.3 wt % (or desirably inthe range of 0.07 to 0.3 wt %) and an average particle size in the rangeof 1 to 5 μm and source materials of additive element including an Sicomponent, a Ca component, a Mg component, a Ba component and a Bcomponent are mixed in prescribed ratios so as to meet the abovedescribed composition in converted values in respective oxides aftersintering. The raw material powder is added with a hydrophilic binder(for example PVA) and water to form a slurry of press formingpreform-use powder. The source materials of additive element can takethe respective following forms such as SiO₂ powder for the Si component;CaCO₃ powder for the Ca component; MgO powder for the a Mg component;BaCO₃ for the Ba component; and H₃BO₃ powder (or used as an aqueoussolution thereof) for the B component. In this case, alumina powder isused that contains a content of the Na component present in surfaceregions of its particles in the range of 0.01 to 0.2 wt % (or desirablyin the range of 0.01 to 0.1 wt %) as Na₂O.

[0121] The slurry is then jet-atomized and dried by a spray method orthe like to form granules of preform-use powder. Thus formed granulesare subjected to rubber press forming, thereby attaining a green whichis an original form of a sintered body. FIG. 11 shows a sketch of aprocess step of rubber press forming. Herein, a rubber mold 300 having acavity 301 the interior of which communicates along its axial directionis employed, and an upper punch 304 is fittingly inserted in an upperopening of the cavity 301. A press pin 303 which defines a form ofthrough hole 6 (FIG. 1) of the insulator 2 not only extends along anaxial direction of the cavity 301 therein, but the lower end of thepress pin 303 is integrally attached to a punch surface of a lower punch302.

[0122] A prescribed amount of the granules of preform-use powder PG arepacked in the cavity in this state and the upper opening of the cavity301 is plugged with the upper punch 304 to closely seal. A hydraulicpressure is applied to the outer side surface of the rubber mold 300 inthe closed state and the granules PG in the cavity 301 is compressedtogether with the rubber mold 300. As a result, a green 305 by the pressas shown in FIG. 12 is obtained. The granules PG is added with water ina ratio of 100 parts by wt of the granules PG to 0.7 to 13 parts by wtof water so that breaking of the granules PG into powder particles isaccelerated and thereafter the pressing is operated.

[0123] The green 305 is mechanically processed by grinding or the likeon its outer side surface into a shape corresponding the insulator 2 ofFIG. 1 and then is subjected to sinter at a temperature in the range of1400 to 1600° C. The sintered body is thereafter applied with a glazeand further receives a heat treatment for finish baking to complete thewhole process.

[0124] In this process, when the preform-use powder slurry is prepared,the pH of the slurry considerably rises since a strongly basic compoundsuch as Na₂O or NaOH that is attached to the Bayer alumina powder(mainly on the surfaces of particles) is dissolved into a solvent. If agranules PG prepared by using such slurry in a high pH state issubjected to the rubber press forming with no other treatment on theslurry, a press forming performance of the granules PG is deterioratedand defects such as cracks C and collapses Y at the inside of theopening brim are apt to be produced. Even if no defects arise directafter the press forming, there is a risk of a trouble such as fracturein grinding or the like for final adjustment of outside dimensions sincea strength of the green 305 itself is reduced.

[0125] Therefore, a proper acidic component, for example a citric acid,boric acid or the like in a proper amount is mixed into the slurry whenbeing prepared and thereby, the pH of the slurry is adjusted to be inthe range of 6 to 10 (or desirably in the range of 7 to 9). Granules PGproduced with the preform-use powder slurry after the pH adjustment hasa very good press forming performance and the green 305 has an improvedproduction yield since the defects as described above are difficult tooccur.

[0126] Below, description will be made upon features spark plug. Thespark plug 100 is mounted to an engine block at its threaded portion 7and used as an ignition source for an air-fuel mixture supplied into acombustion chamber.

[0127] The insulator 2 was produced with Bayer alumina powder of a Nacomponent content as high as in the range 0.07 to 0.65 wt % as Na₂O.Therefore, an insulator produced with the alumina powder also has a Nacomponent content as high as to have conventionally been regarded beyondthe common sense: in the range of 0.07 to 0.5 wt % (part of the Nacomponent has a chance to be lost in the sintering). However, as far asthe Na component content of the insulator 2 is within the abovedescribed range, insulation resistance, mechanical strength and the likeat high temperature is unexpectedly not decreased and therefore, aperformance sufficiently comparable to a conventional insulator of a Nacomponent content lower than that in the range can be obtained.

[0128] As a result of an action and effect described above, while highlyexpensive low soda alumina has conventionally been used in order to keepa Na component content at a low level, in the case of the presentinvention, medium soda and regular aluminas which are much lower in costcan be used as raw material, which entails dramatic decrease inproduction cost of insulator and thereby, of a spark plug 100 as well.

[0129] It should be noted that a spark plug to which insulator of thepresent invention is applicable is not limited to the type shown in FIG.1, but other types may be adopted, for example one as shown in FIG. 5,in which example the top portion of the ground electrode 4 is formed intwo or more ways: tip ends thereof are disposed in an opposed manner tothe side surface of the center electrode 3 to form spark discharge gapsg. In this case, the ground electrode 4 can be arranged in various ways:as shown in FIG. 6(a), two tip ends of the top portion of the groundelectrode 4 are located in an opposite manner to each other close to theside of the center electrode 3, and as shown in FIG. 6(b), three or moretip ends thereof are located around the center electrode 3 so as to bedirected toward the center electrode 3.

[0130] In this case, as shown in FIG. 7, a spark plug 100 may beconstructed as a semi-surface discharge type spark plug in such a mannerthat the fore-end of the insulator 2 is advanced to between the sidesurface of the center electrode 3 and each of the tip ends of the topportion of the ground electrode 4. In this structure, since sparkdischarge arises in a way to run along a surface of the insulator 2,anti-fouling characteristics is improved to compared with an arealdischarge type.

EXAMPLES

[0131] In order to confirm effects of the present invention, thefollowing experiments were conducted.

Example 1

[0132] In order to prepare kinds of raw material powder, kinds of Bayeralumina powder (an average particle diameter of 3.0 μm) of different Nacomponent contents were mixed with various compounds in prescribedratios, wherein a purity and a particle diameter of each of the variouscompounds are as follows: SiO₂ (purity 99.5%, average particle diameter1.5 μm), CaCO₃ (purity 99.9%, average particle diameter 2.0 μm), MgO(purity 99.5%, average particle diameter 2.0 μm), BaCO₃ (purity 99.5%,average particle diameter 1.5 μm), H₃BO₃ (purity 99.0%, average particlediameter 1.5 μm) and ZnO (purity 99.5%, average particle diameter 2.0μm). One hundred parts by wt(weight) of thus prepared powder was furthermixed with 3 parts by wt of PVA as a hydrophilic binder and 103 parts bywt of water in a wet condition to produce a slurry of preform-usepowder. A value of the pH of the slurry was adjusted to 8 by addition ofcitric acid in a proper amount. A total content of Na component and a Nacomponent content in surface regions of particles were measured by theabove described method and an average particle diameter of the aluminapowder was measured using a laser diffraction particle size analyzer.

[0133] Then, the slurries respectively of different compositions weredried by a spray dry method and granules of preform-use powder werethereby prepared, wherein the granules are controlled into particlesizes thereof in the range of 50 to 100 μm by screening. The granuleswere subjected to forming under a pressure of 50 MPa by a rubber pressmethod described using FIG. 11 to obtain a green 305 shown in FIG. 12.The green 305 was subjected to grinding processing on an outer surfacethereof to shape the green into prescribed dimensions of a green 2,which was followed by sintering at prescribed conditions to obtain asintered green 2 made of alumina based insulator whose shape is similarto that of FIG. 1. Sintering conditions were determined in such a mannerthat a sintering period was fixed to be 2 hours but temperature forsintering was changed stepwise at 20° C. temperature differences,apparent densities of the insulators were measured on those obtained inrespective conditions and sintering conditions in which the maximumdensity was attained were adopted.

[0134] Dimensions of the insulators 2 that are expressed according toFIG. 4(a) are as follows: L1=about 60 mm, L2=about 8 mm, L3=about 14 mm,D1=about 10 mm, D2=about 13 mm, D3=about 7 mm, D4=5.5 mm, D5=4.5 mm,D6=4 mm, D7=2.6 mm, t1=1.5 mm, t2=1.45 mm, t3=1.25 mm and tA=1.48 mm.Other dimensions will be shown according to FIG. 1: a length L_(Q) of aportion 2 k projecting toward the backward side of the metal shell 1 ofthe insulator 2 is 25 mm. A length L_(P) measured along a sectionaloutline from a position corresponding to the rear end brim of the metalshell 1 through the corrugation 2 c to the rear end brim of theinsulator 2 is 29 mm.

[0135] Various kinds of spark plugs shown in FIG. 1 were produced usingthus prepared insulator 2, wherein an outer diameter of the threadedportion was 12 mm and a structure was adopted in which the resistor 15was not used and the metal terminal 13 and the center electrode 3 weredirectly connected through a conductive glass layer. The spark plugsreceived the following tests:

[0136] (1) Insulation withstand voltage at 20° C.: the voltage wasmeasured according to the method that is described using FIG. 10,wherein a direct current impulse power supply (peak voltage 35V andpulse width 2 ms) was used as a high-tension power supply,

[0137] (2) Measurement of insulation resistance at 500° C.: themeasurement was conducted at an applied voltage 1000V according to themethod that is described using FIG. 10,

[0138] (3) Withstand voltage test in an actual vehicle: the abovedescribed spark plugs were mounted to a 4 cylinder gasoline engine(piston displacement 2000 cc), the engine was continuously run in athrottle full opening state at an engine revolution number of 6000 rpmwhile a discharge voltage was controlled in the range of 38 to 43 kV andevaluation was conducted by checking whether or not spark penetrationtook place after 50 hours elapsed.

[0139] Besides, strength test pieces were prepared using the samegranules as follows: The granules were press-formed by die pressing(pressure 50 MPa) into a green and sintered in the same conditions as inthe case of the above described insulator. The sintered bodies obtainedwere subjected to grinding in a proper manner to prepare a test piece inthe shape of a prism of 3 mm×3 mm×25 mm. The test piece was used tomeasure a three point bending strength at room temperature (span length20 mm) according to a method described in JIS R1601 (Testing method forflexural strength (modulus of rupture) of high performance ceramics).

[0140] A surface of a test piece after the test was polished and thepolished surface was observed by a scanning electron microscope(magnification 150×). The number of pores, which was revealed on thepolished surface, equal to or larger than 10 μm were counted by means ofimage analysis. A pore surface density which means the number of porescounted per 1 mm² was obtained by dividing a total number of countedpores by an area of a view field in mm². Contents of element componentsof Al, Na, Si, Ca, Mg Ba, Zn and B were analyzed by the ICP method andthe results were converted to contents (in wt %) in the respectiveprescribed oxide forms. A content of Na component in a glassy phase wascalculated by the above described method (wherein EPMA was adopted as amicro-structure analytical method). All the results are shown in Tables1 and 2.

[0141] It is found from the results shown in the tables that theinsulator whose Na component content was in the range of 0.07 to 0.5 wt% was able to obtain an insulation withstand voltage, a strength and awithstand voltage in an actual vehicles comparable to those of anconventional insulator whose Na component content was equal to or lessthan 0.05 wt %, and besides, in the example, values of insulationresistance at 500° C. of spark plugs also showed as high as 200 MΩ ormore.

Example 2

[0142] In order to prepare kinds of raw material powder, kinds of Bayeralumina powder (an average particle diameter of 3.0 μm) of different Nacomponent contents were mixed with various compounds in prescribedratios, wherein a purity and a particle diameter of each of the variouscompounds are as follows: SiO₂ (purity 99.5%, average particle diameter1.5 μm), CaCO₃ (purity 99.9%, average particle diameter 2.0 μm), MgO(purity 99.5%, average particle diameter 2.0 μm). One hundred parts bywt of thus prepared powder was further mixed with 3 parts by wt of PVAas a hydrophilic binder and 103 parts by wt of water in a wet conditionto produce the slurry. A value of the pH of the slurry was adjusted to 8by addition of citric acid in a proper amount, a total content of Nacomponent and a Na component content in surface regions of particleswere measured according to the above described method after cleaning andan average particle diameter of the alumina powder was measured using alaser diffraction particle size analyzer.

[0143] The slurry was used and the same experiments as those in Example1 were conducted, with the results shown in Tables 3 and 4.

[0144] It is found from the results shown in the tables that by usingalumina powder, whose particles have surface regions of a Na componentcontent equal to or less than 0.2 wt %, the insulator was able to havemore excellent results in insulation withstand voltage, insulationresistance at 500° C. and a withstand voltage in real conditions.

Example 3

[0145] One hundred g of each of various kinds of Bayer alumina powder(an average particle diameter of 3.0 μm) of different Na componentcontents was mixed with 100 g of distilled water at 25° C. and themixture was stirred for 10 min, then followed by water washing,dehydration and drying. In order to prepare kinds of raw materialpowder, kinds of Bayer alumina powder which had been cleaned were mixedwith various compounds in prescribed ratios, wherein a purity andparticle diameter of each of the various compounds are as follows: SiO₂(purity 99.5%, average particle diameter 1.5 μm), CaCO₃ (purity 99.9%,average particle diameter 2.0 μm), MgO (purity 99.5%, average particlediameter 2.0 μm), BaCO₃ (purity 99.5%, average particle diameter 1.5 μm)and H₃BO₃ (purity 99.0%, average particle diameter 1.5 μm). One hundredparts by wt of thus prepared powder was further mixed with 3 parts by wtof PVA as a hydrophilic binder and 103 parts by wt of water in a wetcondition to produce a slurry. A value of the pH of the slurry wasadjusted to 8 by addition of citric acid in a proper amount, a totalcontent of Na component and a Na component content in surface regions ofparticles were measured according to the above described method aftercleaning and an average particle diameter of the alumina powder wasmeasured using a laser diffraction particle size analyzer.

[0146] The slurry was used and the same experiments as those in Example1 were conducted, with the results shown in Tables 5 and 6.

[0147] It is found from the results shown in the tables that whenalumina powder whose particles had surface regions of a Na componentcontent less than 0.2 wt % was used, even the sintered bodies, whose Nacomponent content was a little more than 0.5 wt % was able to have stillcomparatively excellent results in insulation withstand voltage,strength, insulation resistance at 500° C. and withstand voltage in anactual vehicle.

Example 4

[0148] In order to prepare kinds of raw material powder, kinds of Bayeralumina powder of almost the same Na component contents, but of averageparticle diameters different from one another were mixed with variouscompounds in prescribed ratios, wherein a purity and particle diameterof each of the various compounds are as follows: SiO₂ (purity 99.5%,average particle diameter 1.5 μm), CaCO₃ (purity 99.9%, average particlediameter 2.0 μm), MgO (purity 99.5%, average particle diameter 2.0 μm),BaCO₃ (purity 99.5%, average particle diameter 1.5 μm), H₃BO₃ (purity99.0%, average particle diameter 1.5 μm). One hundred parts by wt ofthus prepared powder was further mixed with 3 parts by wt of PVA as ahydrophilic binder and 103 parts by wt of water in a wet condition toproduce the slurry. A value of the pH of the slurry was adjusted to 8 byaddition of citric acid in a proper amount, a total content of Nacomponent and a Na component content in surface regions of particleswere measured according to the above described method after cleaning andan average particle diameter of the alumina powder was measured using alaser diffraction particle size analyzer.

[0149] The slurry was used and the same experiments as those in Example1 were conducted, with the results shown in Tables 7 and 8.

[0150] It is found from the results shown in the tables that as anaverage particle diameter of alumina powder is increased, an optimalsintering temperature is raised.

Example 5

[0151] In order to prepare kinds of raw material powder, kinds of Bayeralumina powder (an average particle diameter of 3.0 μm) of different Nacomponent contents were mixed with various compounds in prescribedratios, wherein a purity and a particle diameter of each of the variouscompounds are as follows: SiO₂ (purity 99.5%, average particle diameter1.5 μm), CaCO₃ (purity 99.9%, average particle diameter 2.0 μm), MgO(purity 99.5%, average particle diameter 2.0 μm). One hundred parts bywt of thus prepared powder was further mixed with 3 parts by wt of PVAas a hydrophilic binder and 103 parts by wt of water in a wet conditionto produce the slurry. A value of the pH of the slurry was adjusted to 8by addition of citric acid in a proper amount, a total content of Nacomponent and a Na component content in surface regions of particleswere measured according to the above described method and an averageparticle diameter of the alumina powder was measured using a laserdiffraction particle size analyzer.

[0152] The slurry was used and the same experiments as those in Example1 were conducted with the results shown in Tables 9 and 10.

[0153] It is found from the results shown in the tables that when Al₂O₃component in the insulator was in the range of 85 to 98 wt %, theinsulator was able to have excellent results in any of insulationwithstand voltage and a strength.

Example 6

[0154] In order to prepare kinds of raw material powder, kinds of Bayeralumina powder (an average particle diameter of 3.0 μm) of different Nacomponent contents were mixed with various compounds in prescribedratios shown in Tables 11 and 12 (wherein contents are shown in oxides),wherein a purity and a particle diameter of each of the variouscompounds are as follows: SiO₂ (purity 99.5%, average particle diameter1.5 μm), CaCO₃ (purity 99.9%, average particle diameter 2.0 μm), MgO(purity 99.5%, average particle diameter 2.0 μm), BaCO₃ (purity 99.5%,average particle diameter 1.5 μm), and ZnO (purity 99.5%, averageparticle diameter 1.5 μm). One hundred parts by wt of thus preparedpowder was further mixed with 3 parts by wt of PVA as a hydrophilicbinder and 103 parts by wt of water in a wet condition to produce apreform-use powder slurry.

[0155] A value of the pH of the slurry was adjusted to 8 by addition ofcitric acid or H₃BO₃ as acidic component. Tables 1 land 12 show additiveamounts of respective acidic components in wt % relative to totalweights of respective raw material powder masses (a weight of each rawmaterial powder mass without the acidic component).

[0156] The slurrys were dried by a spray dry method to produce granuleseach with a spherical shape. Sizes of the granules were controlled byscreening so as to be in the range of 50 to 100 μm in particle diameter.The sieved granules were press-formed by the rubber press methoddescribed using FIG. 11 under a pressure of 50 MPa to prepare a green305 with the shape shown in FIG. 12. Dimensions shown in FIG. 12 are asfollows: 1₁=85 mm, Δ₁=19 mm, Δ₂=9 mm, Δ₃=18 mm, δ₁=4.8 and δ₂=3.1 mm. Anobtained green was immersed in a defect inspection liquid (suspension ofred pigment in kerosene) and taken out from the liquid. On thisoccasion, if there are defects such as cracks on the surface of thegreen 305 or collapses at an inner peripheral portion of the openingbrim thereof, the inspection liquid is impregnated into the bulk andthereby no coloring in the appearance is produced. Hence, a degree ofdefect occurrence can visually be recognized observing coloring on thesurface. Evaluation was expressed in three ways on the basis of degreesof defect occurrence; ◯ was used as an evaluation result in a case whereabsolutely no defect was recognized, Δ was used as an evaluation resultin a case where while defects were recognized, the occurrence wasextremely slight and X was used as an evaluation result in a case wheredefects arose very much.

[0157] Green for strength test in a plate of 12 mm×8 mm×80 mm wereproduced using a metal mold press forming (under a pressure of 50 MPa)using the same granules and three point bending strength (a span lengthwas 50 mm) of each test mass was measured at room temperature, whichresults are shown in Tables 11 and 12.

[0158] It is found from the tables that when any of citric acid andH₃BO₃ was used, if a value of the pH of a preform-use powder slurry wasadjusted in the range of 6 to 10 as well, and especially in the range of7 to 9, defects were hard to arise and a bending strength was alsoimproved.

Industrial Applicability

[0159] According to the present invention, as described above, aluminapowder in use is one that contains a high content range of Na componentwhich is regarded as being beyond the common sense: in a concrete mannerof description, in the range of 0.07 to 0.65 wt % as Na₂O and thealumina powder in use further has a Na component content present in thesurface regions of particles of the alumina powder in the range of 0.01to 0.2 wt %. With use of such alumina powder, an insulation resistance,mechanical strength or the like at high temperature of insulator fromthe aluminum powder is not decreased to an extent which would otherwisebe expected and there can be obtained an insulator of the presentinvention showing a performance comparable to a conventional insulatorof a Na component content lower than that of the insulator of thepresent invention. Besides, a spark plug using the insulator of thepresent invention can secure an insulation resistance equal to or higherthan 200 MΩ, which has conventionally been regarded as impossible in theNa component content range, wherein the insulating resistance ismeasured in conditions that the entire spark plug is heated at aconstant temperature of 500° C. and a current is made to pass throughthe spark plug between the terminal metal member and the metal shell atthe temperature. Besides, since medium soda and regular soda aluminawhich are much lower in cost than conventional low conventional can beused instead of the conventional low soda alumina, dramatic costreduction for insulator and thereby, also for a spark plug using thesame can be of reality. TABLE 1 Na₂O in Used alumina powder Na insintered Total Surface Composition (wt %) glassy Sintering Test piecemass Na₂O Na₂O Av. Particle Others phase conditions No. (wt %) (wt %)(wt %) diam. (μm) Al₂O₃ SiO₂ CaO MgO {circle over (1)} {circle over (2)}(wt %) ° C. × (h) A-1* 0.03* 0.04 0.01 3.0 94.0 2.68 1.79 0.5 BaO 0.7B₂O₃ 0.3 0.33 1550 × 2 A-2* 0.05* 0.07 0.02 3.0 94.0 2.67 1.78 0.5 BaO0.7 B₂O₃ 0.3 0.67 1550 × 2 A-3 0.07 0.10 0.03 3.0 94.0 2.66 1.77 0.5 BaO0.7 B₂O₃ 0.3 0.67 1550 × 2 A-4 0.12 0.17 0.05 3.0 94.0 2.64 1.74 0.5 BaO0.7 B₂O₃ 0.3 1.00 1550 × 2 A-5 0.25 0.35 0.14 3.0 94.0 2.57 1.68 0.5 BaO0.7 B₂O₃ 0.3 1.83 1550 × 2 A-6 0.41 0.63 0.19 3.0 94.0 2.49 1.6 0.5 BaO0.7 B₂O₃ 0.3 2.83 1550 × 2 A-7* 0.53* 0.78 0.23 3.0 94.0 2.43 1.54 0.5BaO 0.7 B₂O₃ 0.3 3.67 1550 × 2 A-8~ 0.7* 1.02 0.36 3.0 94.0 2.35 1.450.5 BaO 0.7 B₂O₃ 0.3 4.52 1550 × 2 A-9* 0.94* 1.44 0.43 3.0 94.0 2.231.33 0.5 BaO 0.7 B₂O₃ 0.3 4.82 1550 × 2 E-1 0.31 0.45 0.16 3.0 94 2.541.65 0.5 BaO 1.0 — 1.81 1550 × 2 E-2 0.33 0.45 0.16 3.0 94 2.53 1.64 0.5BaO 0.5 ZnO 0.5 1.60 1550 × 2 E-3 0.30 0.45 0.16 3.0 94 2.55 1.65 0.5B₂O₃ 0.2 ZnO 0.8 2.00 1550 × 2 E-4 0.25 0.35 0.12 3.5 93 3.45 2.2 0.5B₂O₃ 0.6 — 1.48 1560 × 2 E-5 0.25 0.35 0.12 3.5 93 3.45 2.2 0.5 B₂O₃ 0.3BaO 0.3 1.98 1560 × 2

[0160] TABLE 2 Pore With- Withstand surface stand Insulation voltageTest density Voltage resistance in actual piece (count/ 20° C. 500° C.Strength vehicle No. mm²) (kV/mm) (MΩ) (MPa) test result  A-1* 24 435000 450 ◯  A-2* 36 41 4500 460 ◯ A-3 15 45 3500 480 ◯ A-4 53 40 3000400 ◯ A-5 52 41 1000 410 ◯ A-6 41 42  500 430 ◯  A-7* 36 34  150* 450 X A-8* 15 32  100* 440 X  A-9* 18 31   70* 460 X E-1 49 43 1500 410 ◯ E-237 42 1800 420 ◯ E-3 28 46 1900 450 ◯ E-4 22 48 1600 430 ◯ E-5 36 471800 400 ◯

[0161] TABLE 3 Na₂O in Used alumina powder Na in sintered Total SurfaceComposition (wt %) glassy Sintering Test piece mass Na₂O Na₂O Av.Particle Others phase conditions No. (wt %) (wt %) (wt %) diam. (μm)Al₂O₃ SiO₂ CaO MgO {circle over (1)} {circle over (2)} (wt %) ° C. × (h)B-1 0.01 0.02 0.007 3.0 94.0 3.19 2.0 0.8 — — 0.10 1550 × 2 B-2 0.020.03 0.01 3.0 94.0 3.19 1.99 0.8 — — 0.16 1550 × 2 B-3 0.05 0.07 0.023.0 94.0 3.17 1.98 0.8 — — 0.51 1550 × 2 B-4 0.07 0.10 0.03 3.0 94.03.16 1.97 0.8 — — 0.63 1550 × 2 B-5 0.10 0.14 0.05 3.0 94.0 3.15 1.950.8 — — 0.83 1550 × 2 B-6 0.19 0.27 0.08 3.0 94.0 3.1 1.91 0.8 — — 1.711550 × 2 B-7 0.20 0.28 0.11 3.0 94.0 3.1 1.9 0.8 — — 1.68 1550 × 2 B-80.26 0.38 0.15 3.0 94.0 3.07 1.87 0.8 — — 1.73 1550 × 2 B-9 0.32 0.450.16 3.0 94.0 3.04 1.84 0.8 — — 1.86 1550 × 2 B-10 0.35 0.50 0.18 3.094.0 3.02 1.83 0.8 — — 1.98 1550 × 2 B-11 0.38 0.58 0.20 3.0 94.0 3.011.81 0.8 — — 2.53 1560 × 2 B-12* 0.54* 0.77 0.27 3.0 94.0 2.93 1.73 0.8— — 5.44 1550 × 2 B-13* 0.59* 0.98 0.30 3.0 94.0 2.9 1.71 0.8 — — 6.231550 × 2

[0162] TABLE 4 Pore With- Withstand surface stand Insulation voltageTest density Voltage resistance in actual piece (count/ 20° C. 500° C.Strength vehicle No. mm²) (kV/mm) (MΩ) (MPa) test result B-1 26 46 5500470 ◯ B-2 32 43 5000 460 ◯ B-3 28 43 4000 470 ◯ B-4 25 42 3000 450 ◯ B-518 41 3000 430 ◯ B-6 14 42 2500 420 ◯ B-7 23 43 2000 440 ◯ B-8 16 431800 450 ◯ B-9 21 41  850 430 ◯  B-10 34 41  700 430 ◯  B-11 37 39  500370 ◯  B-12* 26 33  120* 420 X  B-13* 24 34  100* 410 X

[0163] TABLE 5 Na₂O in Used alumina powder Na in sintered Total SurfaceComposition (wt %) glassy Sintering Test piece mass Na₂O Na₂O Av.Particle Others phase conditions No. (wt %) (wt %) (wt %) diam. (μm)Al₂O₃ SiO₂ CaO MgO {circle over (1)} {circle over (2)} (wt %) ° C. × (h)B-14 0.1 0.14 0.02 3.0 94.0 2.65 1.75 0.5 BaO 0.7 B₂O₃ 0.3 1.03 1550 × 2B-15 0.2 0.28 0.05 3.0 94.0 2.6 1.7 0.5 BaO 0.7 B₂O₃ 0.3 0.61 1550 × 2B-16 0.38 0.59 0.06 3.0 94.0 2.51 1.61 0.5 BaO 0.7 B₂O₃ 0.3 1.23 1550 ×2 B-17 0.53 0.75 0.10 3.0 94.0 2.43 1.54 0.5 BaO 0.7 B₂O₃ 0.3 1.77 1550× 2 B-18 0.56 0.80 0.13 3.0 94.0 2.42 1.52 0.5 BaO 0.7 B₂O₃ 0.3 1.921550 × 2

[0164] TABLE 6 Pore With- Withstand surface stand Insulation voltageTest density Voltage resistance in actual piece (count/ 20° C. 500° C.Strength vehicle No. mm²) (kV/mm) (MΩ) (MPa) test result B-14 18 48 3800440 B-15 17 49 3000 420 ◯ B-16 22 45 3200 430 ◯ B-17 27 42 2000 410 ◯B-18 18 44 1800 400 ◯

[0165] TABLE 7 Na₂O in Used alumina powder Na in sintered Total SurfaceComposition (wt %) glassy Sintering Test piece mass Na₂O Na₂O Av.Particle Others phase conditions No. (wt %) (wt %) (wt %) diam. (μm)Al₂O₃ SiO₂ CaO MgO {circle over (1)} {circle over (2)} (wt %) ° C. × (h)C-1 0.14 0.20 0.08 0.5 94.0 2.63 1.73 0.5 BaO 0.7 B₂O₃ 0.3 1.88 1540 × 2C-2 0.14 0.20 0.08 0.7 94.0 2.63 1.73 0.5 BaO 0.7 B₂O₃ 0.3 1.73 1540 × 2C-3 0.14 0.20 0.08 1.0 94.0 2.63 1.73 0.5 BaO 0.7 B₂O₃ 0.3 1.61 1540 × 2C-4 0.14 0.20 0.08 1.8 94.0 2.63 1.73 0.5 BaO 0.7 B₂O₃ 0.3 1.57 1540 × 2C-5 0.13 0.20 0.07 2.5 94.0 2.63 1.74 0.5 BaO 0.7 B₂O₃ 0.3 0.61 1560 × 2C-6 0.13 0.20 0.07 3.0 94.0 2.63 1.74 0.5 BaO 0.7 B₂O₃ 0.3 0.65 1560 × 2C-7 0.13 0.20 0.07 4.0 94.0 2.63 1.74 0.5 BaO 0.7 B₂O₃ 0.3 0.73 1560 × 2C-8 0.13 0.20 0.07 4.6 94.0 2.63 1.74 0.5 BaO 0.7 B₂O₃ 0.3 0.70 1560 × 2C-9 0.12 0.20 0.06 5.0 94.0 2.64 1.74 0.5 BaO 0.7 B₂O₃ 0.3 0.96 1580 × 2 C-10 0.12 0.20 0.06 5.8 94.0 2.64 1.74 0.5 BaO 0.7 B₂O₃ 0.3 0.81 1600 ×2  C-11 0.12 0.20 0.06 8.0 94.0 2.64 1.74 0.5 BaO 0.7 B₂O₃ 0.3 0.72 1600× 2  C-12 0.1 0.20 0.06 10.2 94.0 2.65 1.75 0.5 BaO 0.7 B₂O₃ 0.3 0.671620 × 2

[0166] TABLE 8 Pore With- Withstand surface stand Insulation voltageTest density Voltage resistance in actual piece (count/ 20° C. 500° C.Strength vehicle No. mm²) (kV/mm) (MΩ) (MPa) test result C-1 15 46 2800460 ◯ C-2 12 48 2900 480 ◯ C-3 21 51 3500 460 ◯ C-4 18 47 3600 470 ◯ C-519 47 3500 470 ◯ C-6 23 46 3800 460 ◯ C-7 32 43 3800 420 ◯ C-8 45 443500 400 ◯ C-9 39 43 3600 400 ◯  C-10 51 42 3200 360 ◯  C-11 68 38 3000320 ◯  C-12 92 36 3000 300 ◯

[0167] TABLE 9 Na₂O in Used alumina powder Na in sintered Total SurfaceComposition (wt %) glassy Sintering Test piece mass Na₂O Na₂O Av.Particle Others phase conditions No. (wt %) (wt %) (wt %) diam. (μm)Al₂O₃ SiO₂ CaO MgO {circle over (1)} {circle over (2)} (wt %) ° C. × (h)D-1 0.10 0.20 0.06 3.0 80.0 10.6 6.7 2.6 — — 0.41 1550 × 2 D-2 0.10 0.200.06 3.0 85.0 7.9 5.0 2.0 — — 0.53 1550 × 2 D-3 0.13 0.20 0.06 3.0 92.03.13 2.64 2.1 — — 0.96 1550 × 2 D-4 0.13 0.20 0.06 3.0 95.0 2.13 1.640.6 — — 1.55 1560 × 2 D-5 0.14 0.20 0.06 3.0 97.0 1.53 0.93 0.4 — — 1.821560 × 2 D-6 0.14 0.20 0.06 3.0 98.0 0.93 0.63 0.3 — — 1.96 1580 × 2 D-70.15 0.20 0.06 3.0 99.0 0.52 0.23 0.1 — — 3.21 1600 × 2

[0168] TABLE 10 Pore With- Withstand surface stand Insulation voltageTest density Voltage resistance in actual piece (count/ 20° C. 500° C.Strength vehicle No. mm²) (kV/mm) (MΩ) (MPa) test result D-1 98 37 2000350 ◯ D-2 90 41 2200 410 ◯ D-3 36 47 3200 450 ◯ D-4 22 50 3500 470 ◯ D-553 44 3800 470 ◯ D-6 67 40 2500 460 ◯ D-7 88 35 1800 380 ◯

[0169] TABLE 11 Rubber Bending Test Na₂O in Al₂O₃ SiO₂ CaO MgO Other'scontent Citric acid Press Strength Piece Al₂O₃ content content contentcontent BaO ZnO content pH per- of green No. (%) (%) (%) (%) (%) (%) (%)(ext. %) (−) formance (MPa) 1 0.3 94 2.7 1.8 0.5 0.5 0.5 0.0 10.5 X 2.22 0.3 94 2.7 1.8 0.5 0.5 0.5 0.5 9.3 Δ 3.3 3 0.3 94 2.7 1.8 0.5 0.5 0.51.0 8.8 ◯ 5.2 4 0.3 94 2.7 1.8 0.5 0.5 0.5 1.5 8.1 ◯ 5.5 5 0.3 94 2.71.8 0.5 0.5 0.5 2.0 7.2 ◯ 5.3 6 0.3 94 2.7 1.8 0.5 0.5 0.5 2.5 6.5 Δ 3.57 0.3 94 2.7 1.8 0.5 0.5 0.5 3.0 5.9 X 2.9 8 0.3 94 2.7 1.8 0.5 0.5 0.53.5 5.2 X 2.6

[0170] TABLE 12 Rubber Bending Test Na₂O in Al₂O₃ SiO₂ CaO MgO Other'scontent Boric acid Press Strength Piece Al₂O₃ content content contentcontent BaO ZnO content pH per- of green No. (%) (%) (%) (%) (%) (%) (%)(ext. %) (−) formance (MPa) 1 0.3 93 3.2 2.3 0.5 0.5 0.5 0.0 10.7 X 2.02 0.3 93 3.2 2.3 0.5 0.5 0.5 1.0 10.1 X 3.1 3 0.3 93 3.2 2.3 0.5 0.5 0.52.0 9.4 Δ 5.2 4 0.3 93 3.2 2.3 0.5 0.5 0.5 3.0 8.9 ◯ 5.6 5 0.3 93 3.22.3 0.5 0.5 0.5 4.0 8.2 ◯ 5.3 6 0.3 93 3.2 2.3 0.5 0.5 0.5 5.0 7.6 ◯ 5.47 0.3 93 3.2 2.3 0.5 0.5 0.5 6.0 7.1 ◯ 5.2 8 0.3 93 3.2 2.3 0.5 0.5 0.57.0 6.5 Δ 3.4 9 0.3 93 3.2 2.3 0.5 0.5 0.5 8.0 5.8 X 2.8

1. A spark plug comprising: a metal shell arranged outside a centerelectrode; a ground electrode arranged so as to be opposed to the centerelectrode, an end of the ground electrode being connected to the metalshell; and an insulator, which resides between the center electrode andthe metal shell, and which surrounds the outside of the centerelectrode, wherein the insulator includes alumina as a main componentand further a Na component in the range of 0.07 to 0.5 wt % as a valueconverted into Na₂O, and insulation resistance that is measured bypassing a current through the insulator between a metal terminal and themetal shell while the entire spark plug is held at about 500° C. is 200MΩ or higher.
 2. A spark plug according to claim 1, wherein theinsulator includes an Al component in the range of 95 to 98 wt % as avalue converted into Al₂O₃.
 3. A spark plug according to claim 1 or 2,wherein the insulator includes the Na component in the range of 0.07 to0.5 wt % as a value converted into Na₂O and the Al component in therange of 85 to 98 wt % as a value converted into Al₂O₃ and wherein astructure of the insulator is constructed of an alumina based matrixphase grains of 99 wt % or more in alumina content, as a main phase, anda glassy phase formed in grain boundary regions of the alumina basedmatrix phase particles and Na component content WG_(Na) present in aglassy phase is in the range of 0.4 to 2 wt %.
 4. A spark plug accordingto any of claims 1 to 3, wherein the insulator includes the Na componentin the range of 0.07 to 0.5 wt % as a value converted into Na₂O andfurther includes a K component and a Li component equal to or less than0.2 wt % in total respectively as values converted into K₂O and Li₂O. 5.A spark plug according to claim 4, wherein the insulator includes alkalimetal components other than the Na component in total content equal toor less than 0.05 wt % as respective oxides.
 6. A spark plug accordingto any of claims 1 to 5, wherein the insulator includes the Na componentin the range of 0.07 to 0.5 wt % as a value converted into Na₂O andfurther includes one or more selected from the group consisting of Si,Ca, Mg, Ba, Zn and B components at a total content of 60 wt % or morerespectively as values converted into SiO₂, CaO, MgO, BaO, ZnO and B₂O₃of a remaining weight after excluding a weight as a value converted intoAl₂O₃ of the Al component from a total weight.
 7. A spark plug accordingto claim 6, wherein the insulator includes one or more selected from thegroup consisting of Si, Ca and Mg components at a total content of 60 wt% or more respectively as values converted into SiO₂, CaO and MgO of aremaining weight after excluding a weight as a value converted intoAl₂O₃ of the Al component from a total weight.
 8. A spark plug accordingto claim 6 or 7, wherein the insulator includes at least one of the Bacomponent and the B component in the range of 0.2 to 1.2 wt % in totalcontent as oxides.
 9. A spark plug according to any of claims 1 to 8,wherein the number of pores each having a size equal to or larger than10 μm that are observed in a sectional structure of the insulator isequal to or less than 100 as average counts per 1 mm² of the section.10. A spark plug according to any of claims 1 to 9, wherein as rawmaterial of the insulator, alumina powder, whose Na component content isas high as 0.07 to 0.65 wt % as a value converted into Na₂O, and in thesurface regions of whose particles a Na component in the range of 0.01to 0.2 wt % as a value converted into Na₂O is included is used.
 11. Aspark plug according to any of claims 1 to 10, wherein the insulator hasan insulation withstand voltage of 35 kV/mm or higher at 20° C. 12.Alumina based insulator for a spark plug, the insulator including a Nacomponent in the range of 0.07 to 0.5 wt % as a value converted intoNa₂O and having an insulation withstand voltage of 35 kV/mm or higher at20° C.
 13. Alumina based insulator for a spark plug, the insulatorincluding alumina as a main component and further including the Nacomponent in the range of 0.07 to 0.5 wt % as a value converted intoNa₂O, wherein the Al component is included in the range of 95 to 98 wt %as a value converted into Al₂O₃.
 14. Alumina based insulator for a sparkplug, the insulator including alumina as a main component and furtherincluding the Na component in the range of 0.07 to 0.5 wt % as a valueconverted into Na₂O, wherein the Al component is included in the rangeof 85 to 98 wt % as a value converted into Al₂O₃, wherein a structure ofthe insulator constructed of an alumina based matrix phase grains of 99wt % or more in alumina content, as a main phase, and a glassy phaseformed in grain boundary regions of the alumina based matrix phasegrains in which phase a Na component content WG_(Na) present is in therange of 0.4 to 2 wt %.
 15. Alumina based insulator for a spark plug,the insulator including alumina as a main component, further includingthe Na component in the range of 0.07 to 0.5 wt % as a value convertedinto Na₂O and still further including a K component and a Li componentin total content equal to or less than 0.2 wt % respectively as valuesconverted into K₂O and Li₂O.
 16. Alumina based insulator for a sparkplug, the insulator including alumina as a main component, furtherincluding the Na component in the range of 0.07 to 0.5 wt % as a valueconverted into Na₂O and still further including one or more selectedfrom the group consisting of Si, Ca, Mg, Ba, Zn and B components at atotal content of 60 wt % or more respectively as values converted intoSiO₂, CaO, MgO, BaO, ZnO and B₂O₃ of a remaining weight after excludinga weight as a value converted into Al₂O₃ of the Al component from atotal weight.
 17. Insulator according to any of claims 12 to 16 in whosesectional structure the number of pores each having a size equal to orlarger than 10 μm that are observed is equal to or less than 100 asaverage counts per 1 mm² of the section
 18. Insulator according to anyof claims 12 to 17 as raw material of which, alumina powder, whose Nacomponent content is as high as 0.07 to 0.65 wt % as a value convertedinto Na₂O, and in the surface regions of whose particles a Na componentin the range of 0.01 to 0.2 wt % as a value converted into Na₂O isincluded is used.
 19. A production process for alumina based insulatorfor a spark plug, wherein alumina powder, whose Na component content isas high as 0.07 to 0.65 wt % as a value converted into Na₂O, and in thesurface regions of whose particles a Na component in the range of 0.01to 0.2 wt % as a value converted into Na₂O is included is used, rawmaterial powder including the alumina powder as a main component isformed into a prescribed insulator shape as a green and the green issintered, whereby an insulator which includes alumina as a maincomponent, and further includes a Na component in the range of 0.07 to0.5 wt % as a value converted into Na₂O is obtained.
 20. A productionprocess for insulator according to claim 19, wherein the alumina powderis powder produced by a Bayer process.
 21. A production process forinsulator according to claim 19 or 20, wherein raw material powder to beused is prepared by mixing additive element based raw materialsincluding one or more selected from the group consisting of Si, Ca, Mg,Ba, Zn and B components at a total content in the range of 0.1 to 15parts by weight respectively as values converted into SiO₂, CaO, MgO,BaO, ZnO and B₂O₃ into 85 to 98 parts by weight of alumina powder.
 22. Aproduction process for insulator according to any of claims 19 to 21,wherein particles of the alumina powder are in the range of 1 to 5 μm asaverage particle size.
 23. A production process for insulator accordingto any of claims 19 to 22, wherein the alumina powder in use includes atotal content of alkali metal components other than a Na component whichcomponents are unavoidably included is equal to or less than 0.05 wt %as oxide.
 24. A production process for insulator according to any ofclaims 19 to 23, wherein not only is a binder in a prescribed amountmixed in raw material powder to prepare preform-use powder but a properacidic component is added to the preform-use powder to adjust a pH valueof the preform-use powder so as to be lowered, thereafter, thepreform-use powder after the pH adjustment is subjected to press formingto produce a green and then the green is sineterd to obtain aninsulator.
 25. A production process for insulator according to claim 24,wherein a pH value of the preform-use powder is adjusted in the range of6 to
 10. 26. A production process for insulator according to claim 24,or 25, wherein a hydrophilic binder is used as the binder.
 27. Aproduction process for insulator according to claim 26, comprising thesteps of: adding a water based solvent and a hydrophilic binder in aprescribed amount to the raw material powder: mixing the solvent and thepowder with each other to form slurry; adding the acidic component tothe slurry to adjust a pH value of the slurry so as to be in the rangeof 6 to 10; jet-atomizing and drying the slurry to produce granules ofthe preform-use powder; and press-forming the granules to obtain thegreen.
 28. A production process for insulator according to claim 27,wherein 100 parts by weight of press forming granules is added with 0.5to 2.0 parts by weight of water and after the addition of water, thepress forming granules are subjected to press forming.
 29. A productionprocess for insulator according to any of claims 24 to 28, wherein theacidic component or acidic components are one or more selected from thegroup consisting of inorganic acids such as boric acid, colloidalsilica, carbonic acid and phosphoric acid; organic acids such as citricacid, oxalic acid, tartaric acid and acetic acid; and a salt of a weakbase and a strong acid such as ammonium sulfate and ammonium nitrate.